Digital coherent receiving apparatus

ABSTRACT

A digital coherent receiving apparatus includes a first oscillator for outputting a local light signal of a fixed frequency, a hybrid unit mixing the local light signal with a light signal received by a receiver, a second oscillator for outputting a sampling signal of a sampling frequency, a converter for converting the mixed light signal into digital signal synchronizing with the sampling signal, a waveform adjuster for adjusting a waveform distortion of the converted digital signal, a phase adjustor for adjusting a phase of the digital signal adjusted by the waveform adjustor, a demodulator for demodulating the digital signal adjusted by the phase adjuster, and a phase detector for detecting a phase of the digital signal adjusted by the phase adjuster, and a control signal output unit for outputting a frequency control signal on the basis of the detected phase signal to the second oscillator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2009-150124, filed on Jun. 24,2009, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are a digital coherent receiver.

BACKGROUND

Along with an increase in the internet traffic, a larger capacity in anoptical communication system of a trunk line system is demanded, andresearch and development are carried out on an optical transmitterreceiver capable of transmitting a signal exceeding 100 [Gbit/s] perwavelength. However, in the optical communication, when a bit rate perwavelength is increased, a degradation in a signal quality becomes largebecause of an decrease in the bearing force for Optical Signal NoiseRatio (OSNR), the wavelength dispersion in a transmission path, thepolarized wave mode dispersion, or the waveform distortion from annonlinear effect or the like.

For this reason, in recent years, a digital coherent reception systemhaving the OSNR bearing force and the waveform bearing force in thetransmission path attracts attention (for example, see D. Ly-Gagnon etal, IEEE JLT, vol. 24, pp. 12-21, 2006). Also, in contrast to a systemfor the direct detection by assigning ON/OFF of a light intensity tobinary signals in a related art, according to the digital coherentreception system, a light intensity and phase information are extractedthrough the coherent reception system. Then, the extracted intensity andphase information are quantized by an ADC (Analog/Digital Converter) anddemodulated by a digital signal processing circuit (for example, see F.M. Gardner, “A BPSK/QPSK timing-error detector for sampled receivers”,IEEE Trans. Commun., vol. COM-34, pp. 423-429, May 1986).

However, according to the related art technology, when the frequency ofthe local light in the digital coherent receiver varies with respect tothe frequency of the optical light transmitted from the transmitter, theoptical signal cannot be digitally demodulated at a satisfactoryprecision in the digital coherent receiver. For this reason, a problemoccurs that a communication quality is degraded.

SUMMARY

According to an aspect of the invention, A digital coherent receivingapparatus includes a receiver for receiving a light signal, a firstoscillator for outputting a local light signal of a fixed frequency, ahybrid unit mixing the local light signal output from the firstoscillator with the light signal received by the receiver, a secondoscillator for outputting a sampling signal of a sampling frequency, thesecond oscillator changing the sampling frequency in response to aninput a frequency control signal, a converter for converting the mixedlight signal into digital signal by sampling the mixed light signalsynchronizing with the sampling signal, a waveform adjuster foradjusting a waveform distortion of the digital signal converted by theconvertor, a phase adjustor for adjusting a phase of the digital signaladjusted by the waveform adjustor, a demodulator for demodulating thedigital signal adjusted by the phase adjuster, a phase detector fordetecting a phase of the digital signal adjusted by the phase adjuster;and a control signal output unit for outputting a frequency controlsignal on the basis of the detected phase signal to the secondoscillator.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration example 1 of a digitalcoherent receiver;

FIG. 2 is a block diagram showing a configuration example 2 of thedigital coherent receiver;

FIG. 3 is a block diagram showing a configuration example 3 of thedigital coherent receiver;

FIG. 4 is a block diagram showing a configuration example 4 of thedigital coherent receiver;

FIG. 5 is a block diagram showing a specific example 1 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 6 is a block diagram showing a specific example 2 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 7 is a block diagram showing a specific example 1 of a phaseadjuster;

FIG. 8 is a block diagram showing a specific example 2 of the phaseadjuster;

FIG. 9 is a block diagram showing a specific example of a first DLF;

FIG. 10 is a block diagram showing a specific example of a second DLF;

FIG. 11 is a block diagram showing a specific example 3 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 12 is a block diagram showing a specific example 4 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 13 is a block diagram showing a specific example 1 of acompensation circuit;

FIG. 14 is a block diagram showing a specific example 2 of thecompensation circuit;

FIG. 15 is a block diagram showing a specific example 1 of the phasecontrol circuit shown in FIG. 4;

FIG. 16 is a block diagram showing a specific example 2 of the phasecontrol circuit shown in FIG. 4;

FIG. 17 is a block diagram showing a specific example 3 of the phasecontrol circuit shown in FIG. 4;

FIG. 18 is a block diagram showing a specific example 4 of the phasecontrol circuit shown in FIG. 4;

FIG. 19 is a block diagram showing a specific example 1 of afrequency/phase compensation circuit;

FIG. 20 is a block diagram showing a specific example 2 of thefrequency/phase compensation circuit;

FIG. 21 is a block diagram showing a specific example 5 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 22 is a block diagram showing a specific example 6 of the phasecontrol circuit shown in FIGS. 1 to 3;

FIG. 23 is a block diagram showing a configuration example of a phasedetector used for a phase detection unit;

FIG. 24 is a graph showing a sensitivity correction by a phase detectorof a sensitivity correction type (single-sided correction);

FIG. 25 is a graph showing a sensitivity correction by a phase detectorof a sensitivity correction type (two-sided correction);

FIG. 26 is a block diagram showing a configuration example of asensitivity monitor phase detector (single-sided monitor);

FIG. 27 is a block diagram showing a configuration example of asensitivity monitor phase detector (two-sided monitor);

FIG. 28 is a block diagram showing a configuration example of a phasedetection unit of a sensitivity selection correction type;

FIG. 29 is a block diagram showing a configuration example 1 of a phasedetection unit of a diversity addition type;

FIG. 30 is a block diagram showing a configuration example 2 of thephase detection unit of the diversity addition type;

FIG. 31 is a block diagram showing a configuration example 3 of thephase detection unit of the diversity addition type;

FIG. 32 is a block diagram showing a configuration example 4 of thephase detection unit of the diversity addition type;

FIG. 33 is a block diagram showing a specific example of an equalizationfilter (polarized wave dispersion equalization);

FIG. 34 is a block diagram showing a specific example of an equalizationfilter (wavelength dispersion equalization);

FIG. 35 is a block diagram showing a modified example 1 of the digitalcoherent receiver;

FIG. 36 is a block diagram showing a modified example 2 of the digitalcoherent receiver;

FIG. 37 is a block diagram showing a specific example of a frequencydifference detector;

FIG. 38 is a block diagram showing a specific example of a frequencycompensator;

FIG. 39 is a block diagram showing a specific example of an opticaltransmission system;

FIG. 40 is a block diagram showing specific example of a Fouriertransform unit and an inverse Fourier transform unit; and

FIG. 41 shows an operation of the circuit shown in FIG. 40.

DESCRIPTION OF EMBODIMENTS

Hereinafter, with reference to the attached drawings, preferredembodiments of this digital coherent receiver will be described indetail.

(Degradation in Communication Quality Due to Frequency Fluctuation)

First, degradation in a communication quality due to a frequencyfluctuation of a local light source will be described. In aconfiguration in which the wavelength dispersion compensation is carriedout by a waveform distortion compensator of the digital coherentreceiver, in proportion to the size of the wavelength dispersion amountto be compensated in the waveform distortion compensator, a phenomenonis generated that the frequency fluctuation of the local light source istransformed into the sampling phase fluctuation.

A specific description will be given of this phenomenon. Thetransmission signal transmitted from the optical transmitter can berepresented, for example, by the following expression (1). In thefollowing expression (1), s(t) denotes a modulation signal forgenerating the transmission signal. Denoted by j is an imaginary number.Denoted by t is time. Denoted by ω_(o) is a carrier wave frequency ofthe light.

[Expression 1]

s(t)exp(jω₀t)  (1)

A transfer function of the transmission path dispersion can berepresented, for example, by the following expression (2). In thefollowing expression (2), D denotes wavelength dispersion. V_(L) denotesa light speed. Denoted by ω are respective frequencies of the baseband.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack & \; \\{\exp\left\lbrack {j\; \frac{\pi \; V_{L}D}{\omega_{0}^{2}}\left( {\omega - \omega_{0}} \right)^{2}} \right\rbrack} & (2)\end{matrix}$

The reception signal distorted by the wavelength dispersion can berepresented by the following expression (3).

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 3} \right\rbrack & \; \\{{{\,^{\bigwedge}S}\left( {\omega - \omega_{0}} \right)}{\exp\left\lbrack {j\; \frac{{\pi V}_{L}D}{\omega_{0}^{2}}\left( {\omega - \omega_{0}} \right)^{2}} \right\rbrack}} & (3)\end{matrix}$

̂S denotes a frequency domain display of the transmission modulationsignal. The local light can be represented by the following expression(4). In the following expression (4), Δω denotes a frequency differencebetween the signal light and the local light.

[Expression 4]

exp[j(ω₀−Δω)t]  (4)

The signal after the coherent reception after the local light and thesignal light shown in the expression (4) are mixed can be represented asthe following expression (5).

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 5} \right\rbrack & \; \\{{{\,^{\bigwedge}S}\left( {\omega - {\Delta\omega}} \right)}{\exp\left\lbrack {j\; \frac{{\pi V}_{L}D}{\omega_{0}^{2}}\left( {\omega - {\Delta\omega}} \right)^{2}} \right\rbrack}} & (5)\end{matrix}$

In the digital coherent receiver, the signal represented by theexpression (5) is quantized by the ADC to carry out the digital signalprocessing. The following expression (6) represents an inverse transferfunction of the transmission path dispersion in a case where thedispersion compensation in the waveform compensation circuit of thedigital signal processing circuit is performed. In the followingexpression (6), AD denotes a deviation of the transmission pathdispersion and the dispersion compensation amount compensated in thewaveform compensation circuit.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 6} \right\rbrack & \; \\{\exp\left\lbrack {{- j}\frac{\; {{\pi V}_{L}\left( {D - {\Delta \; D}} \right)}}{\omega_{0}^{2}}\omega^{2}} \right\rbrack} & (6)\end{matrix}$

The following expression (7) represents the signal after the wavelengthdispersion compensation.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 7} \right\rbrack & \; \\{{{\,^{\bigwedge}S}\left( {\omega - {\Delta\omega}} \right)}{\exp\left\lbrack {j\; \frac{{\pi V}_{L}}{\omega_{0}^{2}}\left( {{\Delta \; D\; \omega^{2}} - {2D\; {{\Delta\omega} \cdot \omega}} + {D\; \Delta \; \omega^{2}}} \right)} \right\rbrack}} & (7)\end{matrix}$

In the expression (7), when a consideration is given on ΔD=0, the signalafter the waveform compensation can be represented by the followingexpression (8).

$\begin{matrix}{\mspace{79mu} \left\lbrack {{Expression}\mspace{14mu} 8} \right\rbrack} & \; \\{{s\left( {t - \; \frac{2{{\pi V}_{L}\left( {D\; \Delta \; \omega} \right)}}{\omega_{0}^{2}}} \right)}{\exp\left\lbrack {{j\Delta\omega}\left( {t - \; \frac{2{{\pi V}_{L}\left( {D\; \Delta \; \omega} \right)}}{\omega_{0}^{2}}} \right)} \right\rbrack}{\exp\left( {j\frac{\pi \; {V_{L}\left( {D\; \Delta \; \omega^{2}} \right)}}{\omega_{0}^{2}}} \right)}} & (8)\end{matrix}$

From the expression (8), because of the frequency drift of the signallight and the local light and the wavelength dispersion compensation, itis understood that a delay of 2πV_(L)DΔω/ω₀ ² is generated. In thismanner, in proportion to the size of the wavelength dispersion amount tobe compensated in the waveform distortion compensator, the frequencyfluctuation of the local light source is transformed into the samplingphase fluctuation, which affects the accuracy of the digitaldemodulation in the subsequent stage.

(Influence on ADC Sampling Frequency Control Processing)

Also, in the digital coherent reception system where the bit rate isequal to or larger than several tens of Gbit/s, the ADC samplingfrequency also becomes equal to or larger than several tens of GHz. Forthis reason, in a case where a digital signal processing circuit isconstructed by using an inexpensive CMOS (Complementary Metal OxideSemiconductor) process, a serial parallel conversion of the ADC samplingsignal is performed so that the operation frequency becomes aboutseveral hundreds of MHz, and the digital signal processing is performedon the paralleled reception signals. In this manner, in a case where thehigh speed sampling is carried out in the ADC, the circuit scale becomeslarger.

EMBODIMENTS

FIG. 1 is a block diagram showing a configuration example 1 of a digitalcoherent receiver. As shown in FIG. 1, a digital coherent receiver 100according to an embodiment is provided with a PBS 111, a local lightsource 112, a PBS 113, a hybrid circuit 121, a hybrid circuit 122,photoelectric converters 131 to 134, a frequency variable oscillator140, a digital conversion unit 150, and a digital signal processingcircuit 160. The digital coherent receiver 100 is a digital coherentreceiver configured to convert a detection result of a signal light froman optical transmission path and a local light into a digital signal toconduct a digital processing.

To the PBS 111 (Polarization Beam Splitter), the signal light sent viathe optical transmission path is input. The PBS 111 separates the inputsignal light into respective polarization axes (which are set as H axisand V axis). The PBS 111 outputs the separated signal light on the Haxis (horizontally polarized wave) to the hybrid circuit 121. Also, thePBS 111 outputs the separated signal light on the V axis (verticallypolarized wave) to the hybrid circuit 122.

The local light source 112 generates the local light to be output to thePBS 113. The PBS 113 separates the local light output from the locallight source 112 into respective the respective polarization axes (whichare set as H axis and V axis). The PBS 113 outputs the separated locallight on the H axis to the hybrid circuit 121. Also, the PBS 113 outputsthe separated local light on the V axis to the hybrid circuit 122.

The hybrid circuit 121 (90° optical hybrid) performs detection on thebasis of the signal light on the H axis output from the PBS 111 and thelocal light output from the PBS 113. The hybrid circuit 121 outputs anoptical signal corresponding to an amplitude and a phase in an I channelof the signal light to the photoelectric converter 131. Also, the hybridcircuit 121 outputs an optical signal corresponding to an amplitude anda phase in a Q channel of the signal light to the photoelectricconverter 132.

The hybrid circuit 122 (90° optical hybrid) performs detection on thebasis of the signal light on the V axis output from the PBS 111 and thelocal light output from the PBS 113. The hybrid circuit 122 outputs anoptical signal corresponding to the amplitude and the phase in the Ichannel of the signal light to the photoelectric converter 133. Also,the hybrid circuit 122 outputs an optical signal corresponding to theamplitude and the phase in the Q channel of the signal light to thephotoelectric converter 134.

Each of the photoelectric converter 131 and the photoelectric converter132 photo electrically converts the light signal output from the hybridcircuit 121 be output to the digital conversion unit 150. Each of thephotoelectric converter 133 and the photoelectric converter 134 photoelectrically converts the light signal output from the hybrid circuit122 to be output to the digital conversion unit 150.

The frequency variable oscillator 140 (oscillation unit) generates avariable frequency clock to be output to the digital conversion unit150. Also, the frequency variable oscillator 140 changes the frequencyof the generated clock on the basis of a control of the digital signalprocessing circuit 160.

The digital conversion unit 150 is provided with ADCs 151 to 154. TheADC 151 digitally samples the signal output from the photoelectricconverter 131. Similarly, the ADCs 152 to 154 respectively digitallysamples the signals output from the photoelectric converters 132 to 134.Also, each of the ADCs 151 to 154 performs the digital sampling insynchronization with the clock output from the frequency variableoscillator 140. Each of the ADCs 151 to 154 outputs the digitallysampled signal to the digital signal processing circuit 160.

The digital signal processing circuit 160 is provided with the waveformdistortion compensation circuit 161 (waveform distortion compensationunit), a phase control circuit 162, and an adaptive equalization typedemodulation circuit 163 (demodulation unit). The waveform distortioncompensation circuit 161, the phase control circuit 162, and theadaptive equalization type demodulation circuit 163 may be realized byone DSP (Digital Signal Processor) or mutually different DSPs.

The waveform distortion compensation circuit 161 compensates a waveformdistortion of the signals output from the ADCs 151 to 154 (distortiongenerated in the optical transmission path). To be more specific, in thewaveform distortion compensation circuit 161, a semi-static transmissionpath waveform distortion component which changes depending on apropagation characteristic fluctuation such as temperature fluctuationis compensated. The waveform distortion compensation circuit 161 outputsthe respective signal in which the waveform distortion is compensated tothe phase control circuit 162. the waveform distortion compensationcircuit 161 may be constructed by one circuit block or may have acascade connection configuration with a plurality of divided waveformdistortion compensation circuit blocks.

The phase control circuit 162 conducts a digital phase compensation onthe respective signals output from the waveform distortion compensationcircuit 161. The phase control circuit 162 outputs the compensatedrespective signals to the adaptive equalization type demodulationcircuit 163. the phase control circuit 162 may be constructed by onecircuit for processing the respective signals from the waveformdistortion compensation circuit 161 in parallel or may be constructed bya plurality of circuits corresponding to the respective signals from thewaveform distortion compensation circuit 161. Also, on the basis of thephases of the respective signals output from the waveform distortioncompensation circuit 161, the phase control circuit 162 controls thefrequency of the clock output by the frequency variable oscillator 140.

The adaptive equalization type demodulation circuit 163 conducts thedemodulation on the respective signals output from the phase controlcircuit 162. Also, the adaptive equalization type demodulation circuit163 performs an adaptive equalization type waveform distortioncompensation on the respective signals output from the phase controlcircuit 162 before the demodulation. To be more specific, the adaptiveequalization type demodulation circuit 163 compensates the waveformdistortion component which is generated in the transmission path andfluctuates at a high speed. the adaptive equalization type demodulationcircuit 163 may be constructed by one circuit block or may have acascade connection configuration with a plurality of adaptiveequalization circuit blocks.

For example, in a case where the ADCs 151 to 154 conduct the digitalsampling at equal to or higher than several tens of GHz, such aconfiguration may be adopted that a multiple PLL (Phase-Locked Loop)using the clock output from the frequency variable oscillator 140 as thereference is provided. Also, the digital coherent receiver 100 shown inFIG. 1 can cope with both a polarized wave multiplex transmission systemmultiplexing transmission signals for every polarized wave axis and anon-polarized wave multiplex transmission with which the polarized wavemultiplexing of the transmission signals is not carried out.

In this manner, as the digital coherent receiver 100 detects the phaseof the signal in the subsequent stage of the waveform distortioncompensation circuit 161, the phase fluctuation generated in thewaveform distortion compensation circuit 161 due to the frequencyfluctuation of the local light source 112 can be detected. Also, bycompensating the detected frequency fluctuation in the former stage ofthe adaptive equalization type demodulation circuit 163 to preciselyconduct the digital demodulation in the adaptive equalization typedemodulation circuit 163, it is possible to improve the communicationquality.

Also, on the basis of the detected phase of the signal in the subsequentstage of the waveform distortion compensation circuit 161, the digitalcoherent receiver 100 controls the sampling phase in the digitalconversion unit 150. To be more specific, the digital coherent receiver100 controls the frequency of the clock oscillated by the frequencyvariable oscillator 140. With this configuration, while the enlargementin the circuit scale is suppressed, it is possible to carry out the highspeed sampling in the digital conversion unit 150. Also, the deviationand wander of the modulation frequency of the optical signal and thesampling frequency in the digital coherent receiver 100 is compensated,and it is possible to reduce the phase compensation amount in thewaveform distortion compensation circuit 161.

Also, the adaptive equalization type demodulation circuit 163 of thedigital coherent receiver 100 compensates the waveform distortionfluctuating at a speed higher than the waveform distortion compensatedin the waveform distortion compensation circuit 161 to carry out thedemodulation. For example, the waveform distortion compensation circuit161 compensates the waveform distortion of a semi-static characteristicwhich changes under the temperature fluctuation or the like. With thisconfiguration, while compensating the phase fluctuation due to thefrequency of the transmission light source and the frequency drift ofthe local light source 112 generated under the temperature fluctuationor the like is compensated in the waveform distortion compensationcircuit 161, it is possible to carry out the high precision waveformdistortion compensation and demodulation in the adaptive equalizationtype demodulation circuit 163.

FIG. 2 is a block diagram showing a configuration example 2 of thedigital coherent receiver. In FIG. 2, a part similar to theconfiguration shown in FIG. 1 is assigned with the similar referencesymbol, and a description thereof is omitted. As shown in FIG. 2, thedigital coherent receiver 100 may be provided with a fixed-frequencyoscillator 211 and a DDS 212 (Direct Digital Synthesizer) instead of thefrequency variable oscillator 140 shown in FIG. 1.

The fixed-frequency oscillator 211 (oscillation unit) generates a fixedfrequency clock to be output to the DDS 212. On the basis of the clockoutput from the fixed-frequency oscillator 211, the DDS 212 generates aclock to be supplied to the digital conversion unit 150 as a samplingcontrol clock. Also, the DDS 212 changes the frequency of the clock tobe generated on the basis of the control of the digital signalprocessing circuit 160. Each of the ADCs 151 to 154 performs the digitalsampling in synchronism with the clock output from the DDS 212.

In this manner, the digital coherent receiver 100 controls the frequencyof the sampling control clock supplied by the DDS. With thisconfiguration, while the enlargement in the circuit scale is suppressed,it is possible to carry out the high speed sampling in the digitalconversion unit 150.

FIG. 3 is a block diagram showing a configuration example 3 of thedigital coherent receiver. In FIG. 3, a part similar to theconfiguration shown in FIG. 1 is assigned with the similar referencesymbol, and a description thereof is omitted. As shown in FIG. 3, thedigital coherent receiver 100 in the case of the non-polarized wavemultiplex system may have a configuration provided with a polarized wavecontroller 311 instead of the PBS 111, the PBS 113, the hybrid circuit122, the photoelectric converters 133 and 134 and the ADCs 153 and 154shown in FIG. 1.

The local light source 112 outputs the generated local light to thepolarized wave controller 311. The polarized wave controller 311controls the polarized wave of the local light output from the locallight source 112 so as to be a polarized wave of the signal lightreceived by the digital coherent receiver 100 (for example, H axis). Thepolarized wave controller 311 outputs the local light in which thepolarized wave is controlled to the hybrid circuit 121. To the hybridcircuit 121, the signal light sent via the optical transmission path andthe local light output from the polarized wave controller 311 are input.such a configuration may be adopted that instead of the frequencyvariable oscillator 140 shown in FIG. 3, the fixed-frequency oscillator211 and the DDS 212 (see FIG. 2) is provided. the polarized wavecontroller 311 may be applied to the signal light sent via the opticaltransmission path instead of the local light.

FIG. 4 is a block diagram showing a configuration example 4 of thedigital coherent receiver. In FIG. 4, a part similar to theconfiguration shown in FIG. 1 is assigned with the similar referencesymbol, and a description thereof is omitted. As shown in FIG. 4, thedigital coherent receiver 100 may be provided with a fixed-frequencyoscillator 411 and a frequency/phase compensation circuit 412 instead ofthe frequency variable oscillator 140.

The fixed-frequency oscillator 411 outputs the fixed frequency clock tothe digital conversion unit 150. Each of the ADCs 151 to 154 performsthe digital sampling in synchronism with the clock output from thefixed-frequency oscillator 411. The phase control circuit 162 detectsthe phases of the respective signals output from the waveform distortioncompensation circuit 161 and outputs the frequency control signal andthe phase control signal to the frequency/phase compensation circuit412.

The frequency/phase compensation circuit 412 (frequency/phasecompensation unit) is provided to the digital signal processing circuit160. The frequency/phase compensation circuit 412 performs the frequencycompensation and the phase compensation on the signal output from theADCs 151 to 154 to compensate the sampling phase. To be more specific,on the basis of the frequency control signal and the phase controlsignal output from the phase control circuit 162, the frequency/phasecompensation circuit 412 compensates the sampling phase of the signaloutput from the ADCs 151 to 154. The frequency/phase compensationcircuit 412 outputs the signal in which the sampling phase iscompensated to the waveform distortion compensation circuit 161.

In this manner, the digital coherent receiver 100 performs the frequencycompensation and the phase compensation on the signal converted into thedigital signal on the basis of the detected phase. With thisconfiguration, it is possible to suppress the influence on the digitalprocessing from the fluctuation of the sampling phase in the digitalconversion unit 150. For this reason, for example, even when such aconfiguration that the digital conversion unit 150 performs the samplingin synchronism with the clock oscillated by the fixed-frequencyoscillator 411 is adopted, it is possible to suppress the influence onthe digital processing from the fluctuation of the sampling phase in thedigital conversion unit 150.

(Specific Example of Phase Control Circuit)

FIG. 5 is a block diagram showing a specific example 1 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 5, with regard to a partof the configuration of the digital coherent receiver 100 shown in FIG.1, the respective signals for I and Q channels and the H and V axes arecollectively illustrated. As shown in FIG. 5, the phase control circuit162 is provided with a phase adjuster 511 (PHA: PHase Adjuster), a phasedetection unit 512 (PD: Phase Detector), a first DLF 513 (Digital LoopFilter), and a second DLF 514.

The phase adjuster 511 (phase compensation unit) compensates the phaseof the signal output from the waveform distortion compensation circuit161 on the basis of the phase control signal output from the first DLF513. The phase adjuster 511 outputs the signal in which the phase iscompensated to the subsequent stage (the adaptive equalization typedemodulation circuit 163). The phase detection unit 512 detects thephase of the signal output from the phase adjuster 511. The phasedetection unit 512 outputs the detected phase signal indicating thephase to the first DLF 513.

The first DLF 513 conducts a signal processing on the phase signaloutput from the phase detection unit 512. The signal processingconducted by the first DLF 513 is, for example, noise removal (low passfilter). The first DLF 513 outputs the signal subjected to the signalprocessing as the phase control signal to the phase adjuster 511. Also,the first DLF 513 outputs the signal subjected to the signal processingto the second DLF 514.

The second DLF 514 performs a signal processing on the signal outputfrom the first DLF 513. The signal processing performed by the secondDLF 514 is, for example, the transform from the phase component into thefrequency component. The second DLF 514 outputs the signal subjected tothe signal processing as the frequency control signal to the frequencyvariable oscillator 140. On the basis of the frequency control signaloutput from the second DLF 514, the frequency variable oscillator 140changes the frequency of the clock to be output.

In this manner, the phase detection unit 512 is provided to thesubsequent stage of the phase adjuster 511 and detects the phase of thesignal compensated by the phase adjuster 511. With this configuration,as the control becomes a feed back control in which the phasecompensation result in the phase adjuster 511 returns from the phasedetection unit 512 to the phase adjuster 511, it is possible to easilyperform the compensation processing in the phase adjuster 511. For thisreason, the phase compensation in the phase adjuster 511 can be carriedout precisely, and it is possible to improve the communication quality.

In a case where the configuration of the phase control circuit 162 shownin FIG. 5 is applied to the digital coherent receiver 100 shown in FIG.2, the second DLF 514 outputs the frequency control signal to the DDS212. On the basis of the frequency control signal output from the secondDLF 514, the DDS 212 changes the frequency of the clock to be generated.

FIG. 6 is a block diagram showing a specific example 2 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 6, a configurationsimilar to the configuration shown in FIG. 5 is assigned with the samereference symbol, and a description thereof is omitted. As shown in FIG.6, the phase detection unit 512 may output the detected phase signalindicating the phase to the first DLF 513 and the second DLF 514. Inthis case, the second DLF 514 conducts the signal processing on thephase signal output from the phase detection unit 512.

FIG. 7 is a block diagram showing a specific example 1 of a phaseadjuster. The phase adjuster 511 shown in FIG. 7 is a specific exampleof a time domain compensation type phase adjuster. As shown in FIG. 7,the phase adjuster 511 is provided with a tap position adjustmentselector 710, a delay elements 721 to 72 n, a tap coefficientcalculation unit 730, multiplication units 741 to 74 n, and an adderunit 750.

To the tap position adjustment selector 710, the signal output from theformer stage of the phase adjuster 511 and an integer part obtained bydividing the phase control signal input to the phase adjuster 511 by thesampling period are input. The tap position adjustment selector 710switches connection paths of the delay elements 721 to 72 n inaccordance with the input integer part.

For example, the tap position adjustment selector 710 switches theconnection paths so that the signal output from the former stage of thephase adjuster 511 (the waveform distortion compensation circuit 161) isinput to the delay element 721. Also, the tap position adjustmentselector 710 switches the connection path so that the output of thedelay element 721 is connected to the input of the delay element 722,the output of the delay element 722 is connected to the input of thedelay element 723, . . . , and the output of the delay element 72(n−1)is connected to the input of the delay element 72 n.

Each of the delay elements 721 to 72 n delays and outputs the inputsignal. To the tap coefficient calculation unit 730, a decimal partobtained by dividing the phase control signal input to the phaseadjuster 511 by the sampling period (phase less than 1 sample) is input.On the basis of the input decimal part, the tap coefficient calculationunit 730 calculates the respective tap coefficients of themultiplication units 741 to 74 n.

For example, the tap coefficient calculation unit 730 calculates the tapcoefficients by sampling a filtering waveform such as a sinc function bythe input phase of the decimal part. Alternatively, the tap coefficientcalculation unit 730 decides the respective tap coefficients on thebasis of a table in which the decimal parts and the respective tapcoefficients are associated with each other. The table in which thedecimal parts and the respective tap coefficients are associated witheach other is previously stored, for example, in a memory of the digitalcoherent receiver 100. The tap coefficient calculation unit 730respectively outputs the calculated respective tap coefficients to themultiplication units 741 to 74 n.

To the multiplication units 741 to 74 n, the output signals of the delayelements 721 to 72 n and the tap coefficients output from the tapcoefficient calculation unit 730 are respectively input. Each of themultiplication units 741 to 74 n multiplies the thus input output signalby the tap coefficient to be output to the adder unit 750. The adderunit 750 adds the respective output signals output from themultiplication units 741 to 74 n to be output to the subsequent stage.

In a case where the signals are input in N parallel with respect to thephase adjuster 511, the delay elements 721 to 72 n are omitted, andinput selectors at a maximum width of the integer part of the phasecontrol signal and FIR (Finite Impulse Response) filters having the sametap coefficient calculated by the tap coefficient calculation unit 730are operated in parallel by N pieces. In this case, the input selectorsare provided by the number of the taps.

FIG. 8 is a block diagram showing a specific example 2 of the phaseadjuster. The phase adjuster 511 shown in FIG. 8 is a specific exampleof a frequency domain compensation type phase adjuster. As shown in FIG.8, the phase adjuster 511 is provided with a Fourier transform unit 811,a rotator transform unit 812, a multiplication unit 813, and an inverseFourier transform unit 814. The Fourier transform unit 811 subjects thesignal input to the phase adjuster 511 to Fourier Transform (FFT: FastFourier Transform) to be transformed into the frequency domain. TheFourier transform unit 811 outputs the signal subjected to the Fouriertransform to the multiplication unit 813.

The rotator transform unit 812 performs the rotator transform processingon the phase control signal output from the first DLF 513 and outputsthe phase shift coefficient obtained through the rotator transformprocessing to the multiplication unit 813. The multiplication unit 813multiplies the signal output from the Fourier transform unit 811 by thephase shift coefficient output from the rotator transform unit 812 andoutputs the multiplied signal to the inverse Fourier transform unit 814.The inverse Fourier transform unit 814 subjects the signal output fromthe multiplication unit 813 to Inverse Fourier Transformation (IFFT:Inverse FFT: Inverse Fast Fourier Transformation) to be output to thesubsequent stage (the adaptive equalization type demodulation circuit163).

FIG. 9 is a block diagram showing a specific example of the first DLF.As shown in FIG. 9, the first DLF 513 is provided with a low pass filter911 (LPF: Low Pass Filter), a multiplication circuit 912, an addercircuit 913, a delay element 914, a multiplication circuit 915, a lowpass filter 916, and an adder circuit 917. To the low pass filter 911,the phase signal output from the phase detection unit 512 is input. Thelow pass filter 911 extracts a low frequency component of the inputphase signal and outputs the extracted signal to the multiplicationcircuit 912 and the multiplication circuit 915.

The multiplication circuit 912 multiplies the signal output from the lowpass filter 911 by a coefficient b to be output to the adder circuit913. The adder circuit 913 adds the signal output from themultiplication circuit 912 with the signal output from the delay element914 to output the added signal as an integral term to the delay element914 and the adder circuit 917. The delay element 914 delays the signaloutput from the adder circuit 913 by one operational clock of the firstDLF and outputs the delayed signal to the adder circuit 913.

The multiplication circuit 915 multiplies the signal output from the lowpass filter 911 by a coefficient a to be output to the low pass filter916. The low pass filter 916 extracts the low frequency component of thesignal output from the multiplication circuit 915 and outputs theextracted signal as a proportional term to the adder circuit 917. Theadder circuit 917 adds the signal of the integral term output from theadder circuit 913 with the signal of the proportional term output fromthe low pass filter 916. The adder circuit 917 outputs the added signalas the phase control signal to the phase adjuster 511.

With the configuration, the phase signal input to the first DLF 513 isconverted as a sum of the proportional term and the integral term havingthe coefficients a and b into the phase control signal. The coefficientsa and b are decided, for example, in accordance with a design and atransmission mode of the digital coherent receiver 100.

The low pass filter 911 operates as a decimation filter for processingthe respective phase signals of the paralleled respective signals (the Iand Q channels and the H and V axes). For example, as a simple example,the low pass filter 911 outputs an average or a total sum of therespective phase signals. it is also possible to adopt a configurationomitting the low pass filter 911.

The low pass filter 916 is provided for suppressing the high frequencynoise component of the phase signal. In some cases, the frequencyfluctuation of the local light source 112 may have a component equal toor higher than several hundreds of kHz. For this reason, in order tominimize the control loop delay, the low pass filter 916 for suppressingthe high frequency noise is inserted only to the proportional term. itis also possible to adopt a configuration omitting the low pass filter916.

FIG. 10 is a block diagram showing a specific example of the second DLF.As shown in FIG. 10, the second DLF 514 is provided with amultiplication circuit 1011, an adder circuit 1012, a delay element1013, a multiplication circuit 1014, an adder circuit 1015, and a lowpass filter 1016. The phase signal input to the second DLF 514 (or thephase control signal input) is input to the multiplication circuit 1011and the multiplication circuit 1014.

The multiplication circuit 1011 multiplies the input signal by acoefficient B to be output to the adder circuit 1012. The adder circuit1012 adds the signal output from the multiplication circuit 101 with thesignal output from the delay element 1013 and outputs the added signalas the integral term to the delay element 1013 and the adder circuit1015. The delay element 1013 delays the signal output from the addercircuit 1012 by one operational clock of the second DLF and outputs thedelayed signal to the adder circuit 1012.

The multiplication circuit 1014 multiplies the input signal by acoefficient A and outputs the multiplied signal as the proportional termto the adder circuit 1015. The adder circuit 1015 adds the signal of theintegral term output from the adder circuit 1012 with the signal of theproportional term output from the multiplication circuit 1014 to beoutput to the low pass filter 1016. The low pass filter 1016 extractsthe low frequency component of the signal output from the adder circuit1015 and outputs the extracted signal as the frequency control signal tothe frequency variable oscillator 140.

With the configuration, the signal input to the second DLF 514 isconverted as a sum of the proportional term and the integral term havingthe coefficients A and B into the frequency control signal. Thecoefficients A and B are decided, for example, in accordance with adesign and a transmission mode of the digital coherent receiver 100.

For example, as shown in FIG. 6, in a case where the phase signal outputfrom the phase detection unit 512 is directly input to the second DLF514, a low pass filter may be provided in the former stage of themultiplication circuit 1011 and the multiplication circuit 1014. The lowpass filter provided in the former stage of the multiplication circuit1011 and the multiplication circuit 1014 conducts the integral operationof the decimation filter for the phase signals and the phaseinformation. Also, the low pass filter 1016 is a low pass filter foravoiding the high frequency noise placed on the clock output from thefrequency variable oscillator 140. it is also possible to adopt aconfiguration omitting the low pass filter 1016.

FIG. 11 is a block diagram showing a specific example 3 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 11, a configurationsimilar to the configuration shown in FIG. 5 is assigned with the samereference symbol, and a description thereof is omitted. In a case wherethe waveform distortion compensation circuit 161 is a circuit forconducting the waveform distortion compensation in the frequency domain,as shown in FIG. 11, a compensation circuit 1111 in which the waveformdistortion compensation circuit 161 and the phase adjuster 511 areintegrally constructed may be provided instead of the waveformdistortion compensation circuit 161 and the phase adjuster 511 shown inFIG. 5. such a configuration may be adopted that instead of thefrequency variable oscillator 140 shown in FIG. 11, the fixed-frequencyoscillator 211 and the DDS 212 (see FIG. 2) are provided.

FIG. 12 is a block diagram showing a specific example 4 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 12, a configurationsimilar to the configuration shown in FIG. 6 is assigned with the samereference symbol, and a description thereof is omitted. In a case wherethe waveform distortion compensation circuit 161 is a circuit forconducting the waveform distortion compensation in the frequency domain,as shown in FIG. 12, instead of the waveform distortion compensationcircuit 161 and the phase adjuster 511 shown in FIG. 6, the compensationcircuit 1111 in which the waveform distortion compensation circuit 161and the phase adjuster 511 are integrally constructed may be provided.such a configuration may be adopted that instead of the frequencyvariable oscillator 140 shown in FIG. 12, the fixed-frequency oscillator211 and the DDS 212 (see FIG. 2) are provided.

FIG. 13 is a block diagram showing a specific example 1 of thecompensation circuit. The compensation circuit 1111 shown in FIGS. 11and 12 is provided, for example, as shown in FIG. 13, with a Fouriertransform unit 1311, a rotator transform unit 1312, a multiplicationunit 1313, a multiplication unit 1314, and an inverse Fourier transformunit 1315.

The Fourier transform unit 1311 performs the Fourier transform on thesignal input to the compensation circuit 1111 to be transformed into thefrequency domain. The Fourier transform unit 1311 outputs the signalsubjected to the Fourier transform to the multiplication unit 1313. Therotator transform unit 1312 performs the rotator transform processing onthe phase control signal output from of the first DLF 513 and outputsthe phase shift coefficient obtained through the rotator transformprocessing to the multiplication unit 1314.

The multiplication unit 1313 multiplies the signal output from theFourier transform unit 1311 by the waveform distortion correctioncoefficient in the frequency domain and outputs the multiplied signal tothe multiplication unit 1314. The waveform distortion correctioncoefficient multiplied in the multiplication unit 1313 is a coefficientdecided in accordance with the waveform distortion of the receptionsignal and is previously stored, for example, in the memory of thedigital coherent receiver 100.

The multiplication unit 1314 multiplies the signal output from themultiplication unit 1313 by the phase shift coefficient output from therotator transform unit 1312 and outputs the multiplied signal to theinverse Fourier transform unit 1315. The inverse Fourier transform unit1315 subjects the signal output from the multiplication unit 1314 to theinverse Fourier transform to be output to the subsequent stage (theadaptive equalization type demodulation circuit 163). such aconfiguration may also be adopted that the multiplication unit 1314 isprovided in the former stage of the multiplication unit 1313. That is,the order for multiplying the waveform distortion correction coefficientand the phase shift coefficient does not make difference in either way.

In this manner, the waveform distortion compensation circuit 161 and thephase adjuster 511 can be realized by the compensation circuit 1111 formultiplying the waveform distortion correction coefficient by the phaseshift coefficient obtained by transforming the phase control signaltransformed by the first DLF 513 into the rotator of the respectivefrequencies in the frequency domain. With this configuration, thewaveform compensation and the phase compensation can be carried out byperforming the Fourier transform by one time. For this reason, it ispossible to realize the miniaturization and speeding up of the circuit.

FIG. 14 is a block diagram showing a specific example 2 of thecompensation circuit. In FIG. 14, a configuration similar to theconfiguration shown in FIG. 13 is assigned with the same referencesymbol, and a description thereof is omitted. The waveform distortioncompensation circuit 161 is a dispersion compensator for compensatingthe wavelength dispersion of the signal, and in a case where thewaveform distortion compensation target in the frequency domain is thewavelength dispersion, the compensation circuit 1111 may have aconfiguration shown in FIG. 14. Herein, the compensation circuit 1111has a configuration omitting the multiplication unit 1313 shown in FIG.13.

The Fourier transform unit 1311 outputs the signal subjected to theFourier transform to the multiplication unit 1314. The rotator transformunit 1312 (rotator transformer) conducts the rotator transformprocessing at the wavelength dispersion compensation amount with thephase control signal output from the first DLF 513 and outputs therotator obtained through the rotator transform processing (thewavelength dispersion and the shift coefficient of the phase) to themultiplication unit 1314. The wavelength dispersion amount at which therotator transform processing is conducted in the rotator transform unit1312 is a coefficient decided in accordance with the wavelengthdispersion of the reception signal and is previously stored, forexample, in the memory of digital coherent receiver 100.

The multiplication unit 1314 multiplies the Fourier transform unit 1311by the rotator output from the rotator transform unit 1312 and outputsthe multiplied signal to the inverse Fourier transform unit 1315. Inthis manner, with use of a state in which the wavelength dispersioncompensation coefficient represented by the expression (6) has anamplitude of 1.0 and has only phase angle information, by performing therotator transform of the wavelength dispersion amount with the phaseangle information on the phase shift coefficient of the phasecompensation processing, the multiplication in the frequency domain canbe conducted by one time.

The processing of the rotator transform unit 1312 can be represented,for example, by the following expression (9). In the followingexpression (9), Δτ denotes a phase control amount in the time domain.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 9} \right\rbrack & \; \\{\exp \left\lbrack {j\left( {{\omega\Delta\tau} - {\frac{{\pi V}_{L}\left( {D - {\Delta \; D}} \right)}{\omega_{0}^{2}}\omega^{2}}} \right)} \right\rbrack} & (9)\end{matrix}$

In this manner, in a case where the waveform distortion compensationcircuit 161 compensates the wavelength dispersion, the compensationcircuit 1111 is provided with the rotator transform unit 1312 fortransforming the wavelength dispersion compensation amount and the phasecontrol signal into rotators having the respective frequencies. Then, asthe compensation circuit 1111 multiplies the signal by the rotatortransformed by the rotator transform unit 1312, it is possible to carryout the waveform compensation and the phase compensation by conductingthe complex multiplication by one time. For this reason, it is possibleto realize the miniaturization and speeding up of the circuit.

FIG. 15 is a block diagram showing a specific example 1 of the phasecontrol circuit shown in FIG. 4. In FIG. 15, a configuration similar tothe configuration shown in FIG. 5 is assigned with the same referencesymbol, and a description thereof is omitted. As shown in FIG. 15, thesecond DLF 514 outputs the signal subjected to the signal processing asthe frequency control signal to the frequency/phase compensation circuit412.

On the basis of the frequency control signal output from the second DLF514, the frequency/phase compensation circuit 412 compensates thesampling phase of the signal from the digital conversion unit 150. Thefrequency/phase compensation circuit 412 outputs the signal in which thesampling phase is compensated to the waveform distortion compensationcircuit 161. The waveform distortion compensation circuit 161compensates the waveform distortion of the signal from thefrequency/phase compensation circuit 412.

In the configuration shown in FIG. 15, such a configuration may beadopted that instead of the waveform distortion compensation circuit 161and the phase adjuster 511, the compensation circuit 1111 in which thewaveform distortion compensation circuit 161 and the phase adjuster 511are integrally constructed (see FIGS. 11 to 14).

FIG. 16 is a block diagram showing a specific example 2 of the phasecontrol circuit shown in FIG. 4. In FIG. 16, a configuration similar tothe configuration shown in FIG. 15 is assigned with the same referencesymbol, and a description thereof is omitted. As shown in FIG. 16, thephase detection unit 512 may output the detected phase signal indicatingthe phase to the first DLF 513 and the second DLF 514. In this case, thesecond DLF 514 conducts the signal processing on the phase signal outputfrom the phase detection unit 512.

In the configuration shown in FIG. 16, such a configuration may beadopted that instead of the waveform distortion compensation circuit 161and the phase adjuster 511, the compensation circuit 1111 in which thewaveform distortion compensation circuit 161 and the phase adjuster 511are integrally constructed is provided (see FIGS. 11 to 14).

FIG. 17 is a block diagram showing a specific example 3 of the phasecontrol circuit shown in FIG. 4. In FIG. 17, a configuration similar tothe configuration shown in FIG. 15 is assigned with the same referencesymbol, and a description thereof is omitted. As shown in FIG. 17, thephase control circuit 162 may have a configuration omitting the phaseadjuster 511 in the configuration shown in FIG. 15. The first DLF 513outputs the signal subjected to the signal processing as the phasecontrol signal to the frequency/phase compensation circuit 412.

The frequency/phase compensation circuit 412 performs the compensationfor the sampling phase on the basis of the frequency control signal fromthe second DLF 514 and also compensates the phase of the signal outputfrom the waveform distortion compensation circuit 161 on the basis ofthe phase control signal output from the first DLF 513. Thefrequency/phase compensation circuit 412 outputs the signal subjected tothe compensation to the waveform distortion compensation circuit 161. Inthis manner, the compensation is conducted in the former stage of thewaveform distortion compensation circuit 161 also with the inclusion ofthe phase fluctuation generated in the waveform distortion compensationcircuit 161.

FIG. 18 is a block diagram showing a specific example 4 of the phasecontrol circuit shown in FIG. 4. In FIG. 18, a configuration similar tothe configuration shown in FIG. 17 is assigned with the same referencesymbol, and a description thereof is omitted. As shown in FIG. 18, thephase detection unit 512 may output the detected phase signal indicatingthe phase to the first DLF 513 and the second DLF 514. In this case, thesecond DLF 514 conducts the signal processing on the phase signal outputfrom the phase detection unit 512.

FIG. 19 is a block diagram showing a specific example 1 of thefrequency/phase compensation circuit. The frequency/phase compensationcircuit 412 shown in FIG. 19 is a specific example of a time domaincompensation type digital frequency/phase compensation circuit. Herein,it is assumed that the oscillation frequency of the fixed-frequencyoscillator 411 is set slightly higher than the reception signal. Asshown in FIG. 19, the frequency/phase compensation circuit 412 isprovided with a frequency phase converter 1910, a parallel conversionunit 1920, a tap coefficient calculation unit 1930, and N pieces of FIRfilters 1940.

The frequency phase converter 1910 converts the output of the second DLF514 from the frequency into the phase for using the output of the secondDLF 514 (the frequency control signal) as the phase control signal. Thefrequency phase converter 1910 is, for example, an integrator. Aninteger part of the signal converted by the frequency phase converter1910 into the phase is output to the parallel conversion unit 1920 andalso deducted as the number of samples where the control is ended in thefrequency phase converter 1910.

On the basis of the integer part of the signal output from the frequencyphase converter 1910, the parallel conversion unit 1920 converts thesignal input to the frequency/phase compensation circuit 412 into theparallel signal. To be more specific, by using the integer part outputfrom the frequency phase converter 1910 as the control signal, theparallel conversion unit 1920 performs the parallel conversion of 1 to N(in a case where the integer part is “0”) or 1 to N+1 (in a case wherethe integer part is “1”) to be output to the subsequent stage.

M pieces (N−1−M) to (N−1) of previous time latest data denoted byreference numeral 1921 is held in a case where the integer part of theoutput from the frequency phase converter 1910 is “0”. Also, M pieces(N−M) to N of the previous time latest data is held in a case where theinteger part of the output from the frequency phase converter 1910 is“1” as the parallel conversion of 1 to N+1 is carried out in theparallel conversion unit 1920.

Also, the parallel conversion unit 1920 generates a clock for performingthe signal processing in the subsequent stage of the parallel conversionunit 1920. To be more specific, the parallel conversion unit 1920generates clocks of 1/N (in a case where the integer part is “0”) or1/(N+1) (in a case where the integer part is “1”) of the sampling clocksof the digital conversion unit 150 to be output to the subsequent stage.In a case where the parallel conversion of 1 to N+1 is performed, theparallel conversion unit 1920 creates clocks so that one clock time ofthe subsequent stage of the parallel conversion unit 1920 becomes theN+1 sample time.

The decimal part converted into the phase by the frequency phaseconverter 1910 is output to the tap coefficient calculation unit 1930.On the basis of the decimal part of the output from the frequency phaseconverter 1910, the tap coefficient calculation unit 1930 calculates therespective tap coefficients which become the sample positions for the Npieces of FIR filters 1940 (0 to N−1). The tap coefficient calculationunit 1930 outputs the calculated respective tap coefficients to therespectively corresponding FIR filters 1940. The processing by the tapcoefficient calculation unit 1930 includes a latency adjustmentequivalent to the parallel conversion unit 1920.

For example, in a case where the frequency difference between thereception signal and the fixed-frequency oscillator 411 is small, thetap coefficients with respect to the N pieces of FIR filters 1940 may beset to be identical.

Each of the N pieces of FIR filters 1940 (0 to N−1) compensates therespective signals output from the parallel conversion unit 1920 on thebasis of the tap coefficients output from the tap coefficientcalculation unit 1930. Each of the FIR filters 1940 (0 to N−1) outputsthe compensated signal to the subsequent stage as N sample paralleleddata.

Also, as in the configuration shown in FIGS. 17 and 18, in a case wherethe output of the first DLF 513 (the phase control signal) is also inputto the frequency/phase compensation circuit 412, an adder circuit 1950for adding the output of the first DLF 513 to the output of thefrequency phase converter 1910 may be provided.

FIG. 20 is a block diagram showing a specific example 2 of thefrequency/phase compensation circuit. In FIG. 20, a configurationsimilar to the configuration shown in FIG. 19 is assigned with the samereference symbol, and a description thereof is omitted. As shown in FIG.20, instead of the tap coefficient calculation unit 1930 and the FIRfilters 1940 shown in FIG. 19, the frequency/phase compensation circuit412 may be provided with a Fourier transform unit 2011, a rotatortransform unit 2012, a multiplication unit 2013, and an inverse Fouriertransform unit 2014.

The parallel conversion unit 1920 outputs the parallel data (N+1 data)subjected to the parallel conversion to the Fourier transform unit 2011.The Fourier transform unit 2011 subjects the signal output from theparallel conversion unit 1920 to the Fourier transform to be transformedinto the frequency domain. To be more specific, in a case where theinteger part of the output from the frequency phase converter 1910 is“0”, the Fourier transform unit 2011 performs the processing by onlyusing the 1 to N-th inputs.

Also, in a case where the integer part of the output from the frequencyphase converter 1910 is “1” and already the FFT segment begins, theFourier transform unit 2011 uses the 1 to (N+1)-th inputs to be input tothe FFT as the continuous sample. Then, until the FFT segment ends, theFourier transform unit 2011 uses all the signals output from theparallel conversion unit 1920. The last FFT inputs are the 1 to(N−1)-th.

In a case where the FFT segment is to begin after this, the Fouriertransform unit 2011 uses the 2 to (N+1)-th inputs to start the FFT andthereafter uses the 1 to N-th inputs. In a case where the FFT segment isabout to end, the Fourier transform unit 2011 uses the 1 to N-th inputs,and the FFT window ends. The Fourier transform unit 2011 outputs thesignal subjected to the Fourier transform to the multiplication unit2013.

The rotator transform unit 2012 performs the rotator transformprocessing on the decimal part of the output from the frequency phaseconverter 1910 and outputs the shift coefficient obtained through therotator transform processing to the multiplication unit 2013. Theprocessing by the rotator transform unit 2012 includes a latencyadjustment equivalent to the parallel conversion unit 1920 and theFourier transform unit 2011.

The multiplication unit 2013 multiplies the signal output from theFourier transform unit 2011 by the shift coefficient output from therotator transform unit 2012 and outputs the multiplied signal to theinverse Fourier transform unit 2014. The inverse Fourier transform unit2014 subjects the signal output from the multiplication unit 2013 to theinverse Fourier transform to be output to the subsequent stage (thewaveform distortion compensation circuit 161).

A phase shift of a decimal part Δτ of the output from the frequencyphase converter 1910 becomes a rotator coefficient exp(jωΔτ) in thefrequency domain. For this reason, the Fourier transform result of theinput signal is multiplied by the rotator coefficient to conduct theinverse Fourier transform so that the phase shift is realized, thefrequency domain processing in the Fourier transform unit 2011, themultiplication unit 2013, and the inverse Fourier transform unit 2014can be commonly used not only as the frequency/phase compensation butalso, for example, as the compensation processing for the wavelengthdispersion.

FIG. 21 is a block diagram showing a specific example 5 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 21, a configurationsimilar to the configuration shown in FIG. 5 is assigned with the samereference symbol, and a description thereof is omitted. As shown in FIG.21, the phase control circuit 162 may be provided with a phase detectionunit 2111 in addition to the configuration shown in FIG. 5. The phasedetection unit 512 detects the phase of the signal output from thewaveform distortion compensation circuit 161 to the phase adjuster 511.The phase detection unit 512 outputs the detected phase signalindicating the phase to the first DLF 513.

The phase detection unit 2111 detects the phase of the signal outputfrom the phase adjuster 511. The phase detection unit 2111 outputs thedetected phase signal indicating the phase to the second DLF 514. Thefirst DLF 513 conducts the signal processing on the phase signal outputfrom the phase detection unit 512 and outputs the signal subjected tothe signal processing to the phase adjuster 511. The second DLF 514conducts the signal processing on the phase signal output from the phasedetection unit 2111. The second DLF 514 outputs the signal subjected tothe signal processing as the frequency control signal to the frequencyvariable oscillator 140.

In this manner, the phase detection unit 512 may have a configuration ofdetecting the phase of the signal before being compensated by the phaseadjuster 511 in the configuration shown in FIG. 5. In this case, thecontrol becomes a feed forward control in which the phase detectionresult by the phase detection unit 512 is output to the subsequent stageof the phase adjuster 511. such a configuration may be adopted thatinstead of the frequency variable oscillator 140 shown in FIG. 21, thefixed-frequency oscillator 211 and the DDS 212 (see FIG. 2) areprovided.

FIG. 22 is a block diagram showing a specific example 6 of the phasecontrol circuit shown in FIGS. 1 to 3. In FIG. 22, a configurationsimilar to the configuration shown in FIG. 6 is assigned with the samereference symbol, and a description thereof is omitted. As shown in FIG.22, the phase detection unit 512 of the phase control circuit 162detects the phase of the signal output from the waveform distortioncompensation circuit 161 to the phase adjuster 511.

In this manner, the phase detection unit 512 may have a configuration ofdetecting the phase of the signal before being compensated by the phaseadjuster 511 in the configuration shown in FIG. 6. In this case, thecontrol becomes the feed forward control in which the phase detectionresult by the phase detection unit 512 is output to the subsequent stageof the phase adjuster 511. such a configuration may be adopted thatinstead of the frequency variable oscillator 140 shown in FIG. 22, thefixed-frequency oscillator 211 and the DDS 212 (see FIG. 2) areprovided.

(Configuration Example of Phase Detection Unit)

FIG. 23 is a block diagram showing a configuration example of a phasedetector used for the phase detection unit 512. A phase detector 2300shown in FIG. 23 is a Gardner-system phase detector (for example, see F.M. Gardner, “A BPSK/QPSK timing-error detector for sampled receivers”mentioned above). As shown in FIG. 23, the phase detector 2300 isprovided with a delay element 2311, a delay element 2312, a subtractionunit 2313, a multiplication unit 2314, a delay element 2321, a delayelement 2322, a subtraction unit 2323, a multiplication unit 2324, andan adder unit 2330. To the phase detector 2300, for example, a signalsubjected to 2× over sampling is input.

An I channel component (H_i or V_i) of the signal input to the phasedetector 2300 is input to the delay element 2311 and the subtractionunit 2313. The delay element 2311 delays the input signal by ½ symbolsand outputs the delayed signal to the delay element 2312 and themultiplication unit 2314. The delay element 2312 delays the signaloutput from the delay element 2311 by ½ symbols to be output to thesubtraction unit 2313.

The subtraction unit 2313 subtracts the signal input to the phasedetector 2300 from the signal output from the delay element 2312 to beoutput to the multiplication unit 2314. The signal output from thesubtraction unit 2313 is a difference between the signals deviated by 1symbol. The multiplication unit 2314 multiplies the ½ symbols deviatedsignal output from the delay element 2311 by a difference between the 1symbol deviated signals output from the subtraction unit 2313 to beoutput to the adder unit 2330.

A Q channel component (H_q or V_q) of the signal input to the phasedetector 2300 is input to the delay element 2321 and the subtractionunit 2323. The delay element 2321 delays the input signal by ½ symbolsand outputs the delayed signal to the delay element 2322 and themultiplication unit 2324. The delay element 2322 delays the signaloutput from the delay element 2321 by ½ symbols to be output to thesubtraction unit 2323.

The subtraction unit 2323 subtracts the signal input to the phasedetector 2300 from the signal output from the delay element 2322 to beoutput to the multiplication unit 2324. The signal output from thesubtraction unit 2323 is a difference between the 1 symbol deviatedsignals. The multiplication unit 2324 multiplies the ½ symbols deviatedsignal output from the delay element 2321 by a difference between the 1symbol deviated signals output from the subtraction unit 2323 to beoutput to the adder unit 2330.

The adder unit 2330 adds the signal output from the multiplication unit2314 with the signal output from the multiplication unit 2324 to beoutput to the subsequent stage. The processing in the adder unit 2330 isconducted on the basis of a symbol rate (=½ down sampling). With thisconfiguration, the signal output from the adder unit 2330 becomes aphase signal where the signal of the ½ symbols deviated phase is a 0cross point.

Herein, it is also conceivable that the Gardner system phase detector2300 shown in FIG. 23 can be used as the phase detection unit 512, thephase detection sensitivity changes because of the wavelength dispersioncompensation error (SD) and the polarized wave mode dispersion shown inthe expressions (6) and (7). In particular, the change in phasedetection sensitivity based on the polarized wave mode dispersion has adependency with respect to the polarized wave rotation state of theoptical fiber.

FIG. 24 is a graph showing a sensitivity correction by a phase detectorof a sensitivity correction type (single-sided correction). In FIG. 24,the horizontal axis represents a phase of the signal input to the phasedetector. The vertical axis represents an amplitude of the phase signaloutput from the phase detector. A relation 2410 represents a relationbetween the phase of the signal and the amplitude of the phase signal ina case where the sensitivity degradation in the phase detector does notexist. A relation 2420 represents a relation between the phase of thesignal and the amplitude of the phase signal in a case where thesensitivity degradation in the phase detector exists.

Typically, as shown in the relation 2410, the phase detector linearlydetects the phase in a range about ±0.15 to 0.2 symbols by using the 0cross point as the center. However, the phase detection sensitivityindicated by an inclination of the phase detection result is degradedbecause of the wavelength dispersion compensation error (ΔD) and thepolarized wave mode dispersion represented by the expressions (6) and(7). For this reason, as shown in the relation 2410, the phase detectionresult has an inclination different from a phase detection resultexpectation value.

This sensitivity degradation adversely affects the phase control loopthrough which the first DLF 513 and the second DLF 514 are inserted. Forthis reason, a phase shift amount x is set in a range where the phasedetector linearly performs the phase detection, and a phase detectionresult α of the input signal is corrected on the basis of a phasedetection result β of the x phase shifted signal (single-sidedcorrection). The correction coefficient is in proportion to 1/(β−α), butas the current phase is close to the origin, the correction coefficientmay be in proportion to 1/β.

FIG. 25 is a graph showing a sensitivity correction by a phase detectorof a sensitivity correction type (two-sided correction). In FIG. 25, apart similar to the part shown in FIG. 24 is assigned with the samereference symbol, and a description thereof is omitted. The phasedetection result α of the input signal may be corrected on the basis ofthe phase detection results β and γ of the x and −x phase shiftedsignals (two-sided correction). The correction coefficient is assumed tobe in proportion to 2/(β−γ) as the current phase is close to the origin.A proportionality coefficient for the correction value is decided on thebasis of the phase shift amount x and may be decided so as to be theinclination of the phase detection result expectation value through themultiplication of the correction coefficient. Also, β of thesingle-sided correction and (α−γ) of the two-sided correction may be setas negative values.

FIG. 26 is a block diagram showing a configuration example of asensitivity monitor phase detector (single-sided monitor). As shown inFIG. 26, a sensitivity monitor phase detector 2600 is provided with aphase detector 2611 and a sensitive monitor unit 2620. To the sensitivemonitor unit 2620, a branched signal input to the sensitivity monitorphase detector 2600 is input. The phase detector 2611 detects the phaseof the input signal and outputs the detected phase signal indicating thephase (α in FIGS. 24 and 25) to the subsequent stage.

The sensitive monitor unit 2620 is provided with an x phase shift unit2621 and a phase detector 2622 (a second phase detector). The x phaseshift unit 2621 shifts the phase of the input signal by the shift amountx. For example, the x phase shift unit 2621 generates a signal in whichthe phase is shifted by the shift amount x through an inter-sampleinterpolation or the like. The x phase shift unit 2621 outputs the phaseshifted signal to the phase detector 2622.

The phase detector 2622 detects the phase of the signal output from thex phase shift unit 2621. The phase detector 2622 is a phase detectorhaving a sensitivity degradation characteristic similar to the phasedetector 2611. The phase detector 2622 outputs the detected phase signalindicating the phase as the sensitivity monitor value (β in FIGS. 24 and25) to the subsequent stage.

Also, in a case where the parallel signals are input to the sensitivitymonitor phase detector 2600, such a configuration may be adopted that anaveraging unit 2612 (Σ) is provided in the subsequent stage of the phasedetector 2611, and the phase signals of the respective signals outputfrom the phase detector 2611 may be averaged by the averaging unit 2612.Also, in a case where the respective signals on the H axis and the Vaxis are input to the sensitivity monitor phase detector 2600, apolarized wave diversity addition may be conducted in the averaging unit2612.

Also, in a case where the parallel signals are input to the sensitivitymonitor phase detector 2600, for example, such a configuration may beadopted that a down sampling unit 2623 is provided in the former stageof the x phase shift unit 2621, and down sampling is conducted inaccordance with a sensitivity fluctuation speed. The sensitive monitorunit 2620 can adopt a configuration of conducting the down sampling asthe sensitive monitor unit 2620 may be operated at a speed where theoperation can follow a state affecting the phase detection sensitivity(polarized characteristic state fluctuation or the like) among the statefluctuations of the optical transmission path.

Also, in a case where the parallel signals are input to the sensitivitymonitor phase detector 2600, such a configuration may be adopted that anaveraging unit 2624 (Σ) is provided in the subsequent stage of the phasedetector 2622, the phase signals of the respective signals output fromthe phase detector 2622 may be averaged by the averaging unit 2624.Also, in a case where the respective signals on the H axis and the Vaxis are input to the sensitivity monitor phase detector 2600, thepolarized wave diversity addition may be conducted in the averaging unit2624. Also, such a configuration may be adopted that a low pass filter2625 is provided in the output stage of the sensitive monitor unit 2620,and wide-area noise of the sensitivity monitor value is suppressed.

In this manner, the sensitive monitor unit 2620 shifts the phase of thesignal and detects the phase of the phase shifted signal, so that it ispossible to monitor the detection sensitivity of the phase detector2611. Also, the x phase shift unit 2621 shifts the phase in a rangewhere the phase detector 2611 linearly detects the phase. With thisconfiguration, the detection sensitivity of the phase detector 2611 canbe monitored accurately.

FIG. 27 is a block diagram showing a configuration example of asensitivity monitor phase detector (two-sided monitor). In FIG. 27, aconfiguration similar to the configuration shown in FIG. 26 is assignedwith the same reference symbol, and a description thereof is omitted. Asshown in FIG. 27, the sensitive monitor unit 2620 of the sensitivitymonitor phase detector 2600 is provided with a −x phase shift unit 2711(a second phase shift unit), a phase detector 2712 (a third phasedetector), and a subtraction unit 2713 in addition to the configurationshown in FIG. 26.

The phase detector 2622 outputs the phase signal to the subtraction unit2713. The −x phase shift unit 2711 shifts the phase of the input signalby a shift amount −x (opposite direction of the shift amount x). Forexample, the −x phase shift unit 2711 generates a signal in which thephase is shifted by the shift amount −x through the inter-sampleinterpolation or the like. The −x phase shift unit 2711 outputs thephase shifted signal to the phase detector 2712.

The phase detector 2712 detects the phase of the signal output from the−x phase shift unit 2711. The phase detector 2712 is a phase detectorhaving a sensitivity degradation characteristic similar to that of thephase detector 2611. The phase detector 2712 outputs the detected phasesignal indicating the phase to the subtraction unit 2713. Thesubtraction unit 2713 subtracts the phase signal output from the phasedetector 2712 from the phase signal output from the phase detector 2622.The subtraction unit 2713 outputs the signal indicating the subtractionresult as the phase signal to the subsequent stage.

In this manner, the sensitive monitor unit 2620 calculates a differencebetween the respective phases of the signal in which the phase isshifted by x and the signal in which the phase is shifted by −x, so thatit is possible to monitor the detection sensitivity of the phasedetector 2611 with respect to the fluctuations in both directions of thephase. Also, the −x phase shift unit 2711 shifts the phase in a rangewhere the phase detector 2611 linearly detects the phase. With thisconfiguration, it is possible to monitor the detection sensitivity ofthe sensitivity monitor phase detector 2600 accurately.

FIG. 28 is a block diagram showing a configuration example of a phasedetection unit of a sensitivity selection correction type. As shown inFIG. 28, a phase detection unit 2800 is provided with equalizationfilters 2811 to 281N, sensitivity monitor phase detectors 2821 to 282N,a selection unit 2830, a selection switch 2840, a sensitivity correctioncoefficient generation unit 2850, and a multiplication unit 2860. Thephase detection unit 2800 is a sensitivity selection type phasedetection unit and can be applied, for example, to the phase detectionunit 512.

The equalization filters 2811 to 281N are equalization filters havingmutually different equalization characteristics (multiplicationcoefficients and the like). To the respective equalization filters 2811to 281N, the I channel component (H_i) and the Q channel component (H_q)included in the H axis of the signal and the I channel component (V_i)and the Q channel component (V_q) included in the V axis of the signalare input. The equalization filter 2811 performs an equalizationprocessing on the respective input signals to be output to thesensitivity monitor phase detector 2821. Similarly, the equalizationfilters 2812 to 281N respectively perform the equalization processing onthe respective input signals to be respectively output to thesensitivity monitor phase detectors 2822 to 282N.

Each of the sensitivity monitor phase detectors 2821 to 282N is, forexample, the sensitivity monitor phase detector 2600 shown in FIG. 26 or27. The sensitivity monitor phase detector 2821 detects the phase of thesignal on the basis of the respective signals output from theequalization filter 2811 and outputs the detected phase signalindicating the phase to the selection switch 2840. Also, the sensitivitymonitor phase detector 2821 outputs the sensitive monitor value to theselection unit 2830.

Similarly, each of the sensitivity monitor phase detectors 2822 to 282Ndetects the phase of the signal on the basis of the respective signalsoutput from the equalization filters 2812 to 281N and outputs thedetected phase signal indicating the phase to the selection switch 2840.Also, each of the sensitivity monitor phase detectors 2822 to 282Noutputs the sensitive monitor value to the selection unit 2830.

On the basis of the sensitivity monitor values output from thesensitivity monitor phase detectors 2821 to 282N, the selection unit2830 selects one of the sensitivity monitor phase detectors 2821 to282N. To be more specific, the selection unit 2830 selects thesensitivity monitor phase detector that outputs the sensitivity monitorvalue whose absolute value is largest among the sensitivity monitorphase detectors 2821 to 282N. In the selection of the sensitivitymonitor phase detector, in order to avoid an influence from noise, thedetection of the largest value of the sensitivity monitor value may havehysteresis.

The selection is made on the basis of the absolute value of thesensitivity monitor value because the satisfactory phase detection maybe carried out on the basis of a negative sensitivity depending on thepolarized wave mode dispersion state of the optical transmission path.The selection unit 2830 notifies the selection switch 2840 of theselected sensitivity monitor phase detector. Also, the selection unit2830 outputs the largest sensitivity monitor value among the sensitivitymonitor values output from the sensitivity monitor phase detectors 2821to 282N to the sensitivity correction coefficient generation unit 2850.

The selection switch 2840 outputs the phase signal output from thesensitivity monitor phase detector which is notified from the selectionunit 2830 among the respective phase signals output from the sensitivitymonitor phase detectors 2821 to 282N to the multiplication unit 2860.

The sensitivity correction coefficient generation unit 2850 is providedwith an inverse number calculation unit 2851 and a multiplication unit2852. The inverse number calculation unit 2851 calculates an inversenumber of the sensitivity monitor value output from the selection unit2830 to be output to the multiplication unit 2852. The multiplicationunit 2852 multiplies the signal output from the inverse numbercalculation unit 2851 by the coefficient and outputs the multiplicationresult as the sensitivity correction coefficient to the multiplicationunit 2860. The coefficient multiplied in the multiplication unit 2852 iscoefficient equivalent to a phase shift amount x in the sensitivitymonitor phase detectors 2821 to 282N (see FIG. 26 or 27).

The multiplication unit 2860 multiplies the phase signal output from theselection switch 2840 by the sensitivity correction coefficient outputfrom the multiplication unit 2852. The multiplication unit 2860 outputsthe multiplied phase signal to the subsequent stage. according to theconfiguration, the sensitivity correction coefficient generation unit2850 conducts the calculation for the inverse number of the sensitivitymonitor value and the coefficient multiplication, but a table referenceconfiguration may be adopted that the sensitivity monitor value isconverted into the sensitivity correction coefficient on the basis of atable in which the sensitivity monitor value and the sensitivitycorrection coefficient are associated with each other. The table inwhich the sensitivity monitor value and the sensitivity correctioncoefficient are associated with each other is previously stored, forexample, in the memory of digital coherent receiver 100.

In this manner, the phase detection unit 2800 performs the equalizationprocessing on the input signals in parallel by the equalization filters2811 to 281N having mutually different equalization characteristics anddetects the phases of the respective signals subjected to theequalization processing. Also, on the basis of the monitor results ofthe respective detection sensitivities of the sensitivity monitor phasedetectors 2821 to 282N, the phase detection unit 2800 selects one of thesensitivity monitor phase detectors 2821 to 282N and outputs the phasesignal indicating the phase detected by the selected phase detector.

With this configuration, the detection result of the phase detectorwhose detection sensitivity is optimal among the sensitivity monitorphase detectors 2821 to 282N can be used in the phase adjuster 511. Forexample, the detection result of the phase detector whose absolute valueof the sensitivity monitor value is largest among the sensitivitymonitor phase detectors 2821 to 282N is used in the phase adjuster 511.With this configuration, the phase is compensated on the basis of thedetection result of the phase detector whose sensitivity degradation issmallest, and it is possible to detect the phase of the signal furtheraccurately. For this reason, it is possible to further improve thecommunication quality.

Also, the phase detection unit 2800 generates the sensitivity correctioncoefficient in proportion to the inverse number of the monitor result ofthe phase detector selected by the selection unit 2830 among therespective monitor results by the sensitivity monitor phase detectors2821 to 282N. Then, the phase detection unit 2800 multiplies the phaseoutput by the selection switch 2840 by the sensitivity correctioncoefficient. With this configuration, the sensitivity degradation in theselected phase detector is corrected, and it is possible to detect thephase of the signal further accurately. For this reason, it is possibleto further improve the communication quality.

FIG. 29 is a block diagram showing a configuration example 1 of a phasedetection unit of a diversity addition type. A phase detection unit 2900shown in FIG. 29 is provided with an H axis phase detector 2911 (a firstphase detector), a V axis phase detector 2912 (a second phase detector),and an adder unit 2920. The phase detection unit 2900 is a diversityaddition type phase detection unit and can be applied, for example, tothe phase detection unit 512.

To the H axis phase detector 2911, the I channel component (H_i) and theQ channel component (H_q) included in the H axis of the signal areinput. The H axis phase detector 2911 detects the phase of the inputsignal and outputs the detected phase signal indicating the phase to theadder unit 2920. To the V axis phase detector 2912, the I channelcomponent (V_i) and the Q channel component (V_q) included in the V axisof the signal are input. The V axis phase detector 2912 detects thephase of the input signal and outputs the detected phase signalindicating the phase to the adder unit 2920.

The adder unit 2920 adds the phase signal output from the H axis phasedetector 2911 with the phase signal output from the V axis phasedetector 2912. The adder unit 2920 outputs the addition result as thephase signal to the subsequent stage.

In this manner, the phase detection unit 2900 detects the phases of therespective signals on the H axis (the first polarized wave) and the Vaxis (the second polarized wave) and adds the detected respectivephases, so that it is cancel the polarized wave dependency of the phasedetection result. Also, it is possible to suppress the noise of thephase detection result.

FIG. 30 is a block diagram showing a configuration example 2 of thephase detection unit of the diversity addition type. In FIG. 30, aconfiguration similar to the configuration shown in FIG. 28 is assignedwith the same reference symbol, and a description thereof is omitted. Aphase detection unit 3000 shown in FIG. 30 is provided with theequalization filters 2811 to 281N, phase detectors 3011 to 301N and 3021to 302N, adder units 3031 to 303N, and a combining unit 3040. The phasedetection unit 3000 is a diversity addition type phase detection unitand can be applied, for example, to the phase detection unit 512.

To each of the equalization filters 2811 to 281N, the I channelcomponent (H_i) and the Q channel component (H_q) included in the H axisof the signal and the I channel component (V_i) and the Q channelcomponent (V_q) included in the V axis of the signal are input. Therespective equalization filters 2811 to 281N perform the equalizationprocessing on the input respective signals.

The equalization filter 2811 outputs the signal on the H axis subjectedto the equalization processing to the phase detector 3011 and the signalon the V axis subjected to the equalization processing to the phasedetector 3021. Similarly, the equalization filters 2812 to 281N outputthe signals on the H axis subjected to the equalization processing tothe phase detectors 3012 to 301N, respectively, and the signals on the Vaxis subjected to the equalization processing to the phase detectors3022 to 302N, respectively.

The phase detector 3011 detects the phase of the signal on the H axisfrom the equalization filter 2811 and outputs the detected phase signalindicating the phase to the adder unit 3031. Similarly, the phasedetectors 3012 to 301N respectively detect the phases of the signals onthe H axis from the equalization filters 2812 to 281N and respectivelyoutput the detected phase signals indicating the phases to the adderunits 3032 to 303N.

The phase detector 3021 detects the phase of the signal on the V axisfrom the equalization filter 2811 and outputs the detected phase signalindicating the phase to the adder unit 3031. Similarly, the phasedetectors 3022 to 302N respectively detect the phases of the signals onthe V axis from the equalization filters 2812 to 281N and respectivelyoutput the detected phase signals indicating the phases to the adderunits 3032 to 303N.

The adder unit 3031 adds the respective phase signals output from thephase detector 3011 and the phase detector 3021 and outputs the additionresult to the combining unit 3040. Similarly, the adder units 3032 to303N respectively adds the respective phase signals output from thephase detectors 3012 to 301N and the phase detectors 3022 to 302N andoutputs the addition result to the combining unit 3040. The combiningunit 3040 performs the diversity combining of the respective phasesignals output from the adder units 3031 to 303N. The combining unit3040 outputs the phase signal subjected to the diversity combining tothe subsequent stage.

In this manner, the phase detection unit 3000 performs the diversityaddition of the phases detected by the phase detectors 3012 to 301N and3022 to 302N and outputs the addition result as the phase signal. Withthis configuration, even when the phase detector has the sensitivitydegradation, it is possible to detect the phase of the signalaccurately. For this reason, as the phase of the signal is compensatedaccurately, and the digital demodulation in the adaptive equalizationtype demodulation circuit 163 is conducted accurately, so that it ispossible to further improve the communication quality.

FIG. 31 is a block diagram showing a configuration example 3 of thephase detection unit of the diversity addition type. In FIG. 31, aconfiguration similar to the configuration shown in FIG. 28 is assignedwith the same reference symbol, and a description thereof is omitted. Asshown in FIG. 31, a phase detection unit 3100 is provided with athreshold determination unit 3110, AND circuits 3121 to 312N, and acombining unit 3130 instead of the selection unit 2830, the selectionswitch 2840, the sensitivity correction coefficient generation unit2850, and the multiplication unit 2860 shown in FIG. 28.

The phase detection unit 3100 is a configuration example of thediversity addition type phase detection unit and can be applied, forexample, to the phase detection unit 512. The sensitivity monitor phasedetector 2821 outputs the detected phase signal indicating the phase tothe AND circuit 3121 and also outputs the sensitive monitor value to thethreshold determination unit 3110. Similarly, the respective sensitivitymonitor phase detectors 2822 to 282N output the detected phase signalsindicating the phases to the AND circuits 3122 to 312N, respectively,and output the sensitive monitor values to the threshold determinationunit 3110.

The threshold determination unit 3110 conducts a threshold determinationon the respective sensitive monitor values output from the sensitivitymonitor phase detectors 2821 to 282N. To be more specific, the thresholddetermination unit 3110 determines whether or not the sensitive monitorvalue output from the sensitivity monitor phase detector 2821 exceeds apredetermined threshold and outputs the determination result to the ANDcircuit 3121.

For example, in a case where the sensitive monitor value output from thesensitivity monitor phase detector 2821 exceeds the predeterminedthreshold, the threshold determination unit 3110 outputs “1” to the ANDcircuit 3121, and in a case where the sensitive monitor value is equalto or smaller than the predetermined threshold, the thresholddetermination unit 3110 outputs “0” to the AND circuit 3121. Similarly,the threshold determination unit 3110 determines whether or not thesensitivity monitor values from the sensitivity monitor phase detectors2822 to 282N exceed a predetermined threshold and outputs thedetermination results to the respective AND circuits 3122 to 312N.

In a case where the determination result output from the thresholddetermination unit 3110 is “1”, the AND circuit 3121 outputs the phasesignal output from the sensitivity monitor phase detector 2821 to thecombining unit 3130. On the other hand, in a case where thedetermination result output from the threshold determination unit 3110is “0”, the AND circuit 3121 does not output the phase signal outputfrom the sensitivity monitor phase detector 2821.

Similarly, in a case where the determination result output from thethreshold determination unit 3110 is “1”, the respective AND circuits3122 to 312N output the phase signals respectively output from thesensitivity monitor phase detectors 2822 to 282N to the combining unit3130. On the other hand, in a case where the determination result outputfrom the threshold determination unit 3110 is “0”, the AND circuits 3122to 312N do not output the phase signals output from the sensitivitymonitor phase detectors 2822 to 282N.

The combining unit 3130 performs the diversity combining of therespective phase signals output from the AND circuits 3121 to 312N. Thecombining unit 3130 outputs the phase signal subjected to the diversitycombining to the subsequent stage. the threshold in the thresholddetermination unit 3110 may be set as X % of the largest sensitivitymonitor value of the sensitivity monitor phase detectors 2821 to 282N, Y% of the average of the monitor values, or a fixed threshold.

In this manner, the phase detection unit 3100 monitors the respectivedetection sensitivities of the sensitivity monitor phase detectors 2821to 282N and performs the diversity combining on the phases detected bythe phase detectors where it is determined that the monitored respectivedetection sensitivities exceed the threshold. Then, the phase detectionunit 3100 outputs the result of the diversity combining as the phasesignal to the subsequent stage. With this configuration, it is possibleto exclude the detection result from the phase detector whosesensitivity is significantly degraded, and it is therefore possible todetect the phase of the signal further accurately. For this reason, itis possible to further improve the communication quality.

FIG. 32 is a block diagram showing a configuration example 4 of thephase detection unit of the diversity addition type. In FIG. 32, aconfiguration similar to the configuration shown in FIG. 31 is assignedwith the same reference symbol, and a description thereof is omitted. Asshown in FIG. 32, a phase detection unit 3200 is provided withsensitivity correction coefficient generation units 3211 to 321N andmultiplication units 3221 to 322N in addition to the configuration shownin FIG. 31.

The phase detection unit 3200 is a configuration example of thediversity addition type phase detection unit and can be applied, forexample, to the phase detection unit 512. The sensitivity monitor phasedetector 2821 outputs the sensitivity monitor value to the thresholddetermination unit 3110 and the sensitivity correction coefficientgeneration unit 3211. Similarly, each of the sensitivity monitor phasedetectors 2822 to 282N outputs the sensitivity monitor value to thethreshold determination unit 3110 and the sensitivity correctioncoefficient generation units 3212 to 321N.

In a case where the determination result output from the thresholddetermination unit 3110 is “1”, the AND circuit 3121 outputs the phasesignal output from the sensitivity monitor phase detector 2821 to themultiplication unit 3221. Similarly, in a case where the determinationresult output from the threshold determination unit 3110 is “1”, therespective AND circuits 3122 to 312N outputs the phase signalsrespectively output from the sensitivity monitor phase detectors 2822 to282N to the multiplication units 3222 to 322N, respectively.

The sensitivity correction coefficient generation unit 3211 generatesthe sensitivity correction coefficient on the basis of the sensitivitymonitor value output from the sensitivity monitor phase detector 2821and outputs the generated sensitivity correction coefficient to themultiplication unit 3221. Similarly, on the basis of the sensitivitymonitor values respectively output from the sensitivity monitor phasedetectors 2822 to 282N, the sensitivity correction coefficientgeneration units 3212 to 321N generate the sensitivity correctioncoefficients and respectively output the sensitivity correctioncoefficients to the multiplication units 3222 to 322N. Each of thesensitivity correction coefficient generation units 3211 to 321N has aconfiguration, for example, similar to the sensitivity correctioncoefficient generation unit 2850 shown in FIG. 28.

The multiplication unit 3221 multiplies the phase signal output from theAND circuit 3121 by the sensitivity correction coefficient output fromthe sensitivity correction coefficient generation unit 3211. Themultiplication unit 3221 outputs the multiplied phase signal to thecombining unit 3130. Similarly, the multiplication units 3222 to 322Nmultiply the phase signals respectively output from the AND circuits3122 to 312N by the sensitivity correction coefficients respectivelyoutput from the sensitivity correction coefficient generation units 3212to 321N. The multiplication units 3222 to 322N output the multipliedphase signals to the combining unit 3130. The combining unit 3130performs diversity combining of the respective phase signals output fromthe multiplication units 3221 to 322N. The combining unit 3130 outputsthe phase signal subjected to the diversity combining to the subsequentstage.

Also, a configuration in which a divider unit 3240 is provided may beadopted. The threshold determination unit 3110 notifies the divider unit3240 of a number M of the sensitive monitors where the sensitivitymonitor values output from the sensitivity monitor phase detectors 2821to 282N exceed the threshold. The combining unit 3130 outputs the phasesignal to the divider unit 3240. The divider unit 3240 divides the phasesignal output from the combining unit 3130 by the number M notified fromthe threshold determination unit 3110 and outputs the division result asthe phase signal to the subsequent stage. With this configuration, thedetection sensitivity of the phase detection unit 3200 can be setconstant.

In this manner, the phase detection unit 3200 generates a sensitivitycorrection coefficient in proportion to the inverse number of themonitor result from the phase detector where it is determined that thedetection sensitivities exceed the threshold among the sensitivitymonitor phase detectors 2821 to 282N. Then, the phase detection unit3200 multiplies the respective phases subjected to the diversityaddition by the sensitivity correction coefficient. With thisconfiguration, the sensitivity degradation in the phase detector whereit is determined that the detection sensitivities exceed the thresholdis corrected, and it is possible to detect the phase of the signalfurther accurately. For this reason, it is possible to further improvethe communication quality.

FIG. 33 is a block diagram showing a specific example of an equalizationfilter (polarized wave dispersion equalization). For the equalizationfilters 2811 to 281N shown in FIGS. 28, 30, 31, and 32, for example,polarized wave dispersion equalization type equalization filters 2811,2812, . . . shown in FIG. 33 can be applied. As shown in FIG. 33, theequalization filter 2811 is provided with a polarized wave rotator 3311,a DGD adder 3321, and a phase shifter 3331.

The polarized wave rotator 3311 rotates the polarized wave axis of therespective signals on the H axis and the V axis input to theequalization filter 2811 and outputs the respective signals where thepolarized wave axis is rotated to the DGD adder 3321. The DGD adder 3321adds DGD (Differential Group Delay) to the respective signals on the Haxis and the V axis output from the polarized wave rotator 3311. The DGDadder 3321 outputs the respective signals to which the DGD is added tothe phase shifter 3331.

The phase shifter 3331 shifts the phases of the respective signals onthe H axis and the V axis output from the DGD adder 3321 to correct thephase convergent point which may be deviated because of the DGDaddition. The phase shifter 3331 outputs the respective signals in whichthe phases are shifted to the subsequent stage. It is also possible toadopt a configuration omitting the phase shifter 3331.

Similarly, the equalization filters 2812 to 281N are respectivelyprovided with polarized wave rotators 3312 to 331N, DGD adders 3322 to332N, and phase shifters 3332 to 333N. The polarized wave rotators 3312to 331N, the DGD adders 3322 to 332N, and the phase shifters 3332 to333N are respectively similar to the polarized wave rotator 3311, theDGD adder 3321, and the phase shifter 3331, and a description thereof isomitted.

The polarized wave rotators 3312 to 331N have mutually differentpolarized wave rotation amounts. Also, the DGD adders 3321 to 332N havemutually different DGDs. Also, the phase shifters 3331 to 333N havemutually different phase shift amounts. With this configuration, theequalization filters 2811 to 281N have mutually different equalizationcharacteristics.

FIG. 34 is a block diagram showing a specific example of an equalizationfilter (wavelength dispersion equalization). For the equalizationfilters 2811 to 281N shown in FIGS. 28, 30, 31, and 32, for example, afilter of a wavelength dispersion equalization type shown in FIG. 34 canbe applied. The equalization filter 2811 is provided with an H axiswavelength dispersion equalizer 3411 and a V axis wavelength dispersionequalizer 3421.

The H axis wavelength dispersion equalizer 3411 equalizes the wavelengthdispersion of the signal on the H axis input to the equalization filter2811 and outputs the signal in which the wavelength dispersion isequalized to the subsequent stage. The V axis wavelength dispersionequalizer 3421 equalizes the wavelength dispersion of the signal on theV axis input to the equalization filter 2811 and outputs the signal inwhich the wavelength dispersion is equalized to the subsequent stage.

Similarly, the equalization filters 2812 to 281N are respectivelyprovided with the H axis wavelength dispersion equalizers 3412 to 341Nand V axis wavelength dispersion equalizers 3422 to 342N. The H axiswavelength dispersion equalizers 3412 to 341N and the V axis wavelengthdispersion equalizers 3422 to 342N are respectively similar to the Haxis wavelength dispersion equalizer 3411 and the V axis wavelengthdispersion equalizer 3421, and a description thereof is omitted. In thismanner, the equalization filters 2811 to 281N have the wavelengthdispersion equalizers corresponding to the respective signals on the Haxis and the V axis. For the equalization filter, not only the polarizedwave dispersion equalization and the wavelength dispersion equalizationare independently applied, but also a combination of those can beapplied.

(Modified Example of Digital Coherent Receiver)

FIG. 35 is a block diagram showing a modified example 1 of the digitalcoherent receiver. In FIG. 35, with regard to a part of a configurationfor the modified example 1 of the digital coherent receiver 100 shown inFIG. 1, the I and Q channels and the H and V axes are collectivelyillustrated. In FIG. 35, a configuration similar to the configurationshown in FIG. 5 is assigned with the same reference symbol, and adescription thereof is omitted. As shown in FIG. 35, the digitalcoherent receiver 100 is provided with a frequency compensator 3511(frequency compensation unit) and a frequency difference detector 3512(frequency difference detection unit) instead of the phase adjuster 511shown in FIG. 5.

The digital conversion unit 150 outputs the digitally converted signalto the frequency compensator 3511. On the basis of the rotation controlsignal output from the first DLF 513, the frequency compensator 3511compensates the frequency of the signal output from the digitalconversion unit 150. The frequency compensator 3511 outputs the signalin which the frequency is compensated to the waveform distortioncompensation circuit 161. The waveform distortion compensation circuit161 compensates the waveform distortion of the signal output from thefrequency compensator 3511.

The phase detection unit 512 detects the phase of the signal output fromthe waveform distortion compensation circuit 161. The phase detectionunit 512 outputs the detected phase signal indicating the phase to thesecond DLF 514. The second DLF 514 performs the signal processing on thesignal output from the phase detection unit 512 and outputs the signalsubjected to the signal processing as the frequency control signal tothe frequency variable oscillator 140.

The frequency difference detector 3512 detects the frequency differenceof the signal output from the waveform distortion compensation circuit161. The frequency difference detector 3512 outputs the frequencydifference signal indicating the frequency difference between thedetected reception light and the local light to the first DLF 513. Thefirst DLF 513 performs the signal processing on the frequency differencesignal output from the frequency difference detector 3512. The first DLF513 outputs the signal subjected to the signal processing as therotation control signal to the frequency compensator 3511. Such aconfiguration may be adopted that instead of the frequency variableoscillator 140 shown in FIG. 35, the fixed-frequency oscillator 211 andthe DDS 212 (see FIG. 2) are provided.

In this manner, the digital coherent receiver 100 detects the frequencydifference between the reception light and the local light received inthe subsequent stage of the waveform distortion compensation circuit 161and compensates the detected frequency difference fluctuation throughthe frequency compensation in the former stage of the waveformdistortion compensation circuit 161 to suppress the phase fluctuationgenerated in the output of the waveform distortion compensation circuit161 due to the frequency fluctuation of the local light source 112, sothat it is possible to accurately carry out the digital demodulation inthe adaptive equalization type demodulation circuit 163. For thisreason, it is possible to improve the communication quality.

FIG. 36 is a block diagram showing a modified example 2 of the digitalcoherent receiver. In FIG. 36, a configuration similar to theconfiguration shown in FIG. 35 is assigned with the same referencesymbol, and a description thereof is omitted. As shown in FIG. 36, thefrequency difference detector 3512 may detect a frequency difference ofthe signal in the subsequent stage of the frequency compensator 3511.Such a configuration may be adopted that instead of the frequencyvariable oscillator 140 shown in FIG. 36, the fixed-frequency oscillator211 and the DDS 212 (see FIG. 2).

FIG. 37 is a block diagram showing a specific example of the frequencydifference detector. The frequency difference detector 3512 shown inFIGS. 35 and 36 is provided, for example, as shown in FIG. 37, withcomputation units 3711 to 3713 and 3721 to 3723, and an adder unit 3730.With regard to the signal on the H axis input to the frequencydifference detector 3512 (set as X), the computation unit 3711 computesX⁴/|X|⁴ and computation result to the computation unit 3712.

The computation unit 3712 computes arg( ) with respect to thecomputation result output from the computation unit 3711 to be convertedinto the phase information and outputs the computation result to thecomputation unit 3713. The computation unit 3713 performs a computationof the following expression (10) on the computation result output fromthe computation unit 3712 and outputs the computation result to theadder unit 3730.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 10} \right\rbrack & \; \\{\frac{1 - Z^{{- 2}n}}{4n} \cdot \frac{1}{2}} & (10)\end{matrix}$

With regard to the signal on the V axis input to the frequencydifference detector 3512 (set as X), the computation unit 3721 computesX⁴/|X|⁴ and outputs the computation result to the computation unit 3722.The computation unit 3722 computes arg( ) with respect to thecomputation result output from the computation unit 3721 to be convertedinto the phase information and outputs the computation result to thecomputation unit 3723.

The computation unit 3723 performs the computation of the expression(10) on the computation result output from the computation unit 3722 andoutputs the computation result to the adder unit 3730. The adder unit3730 adds the respective computation results output from the computationunit 3713 and the computation unit 3723 with each other and outputs theaddition result as the frequency difference signal to the subsequentstage.

In the respective signals on the H axis and the V axis which are theinputs of the frequency difference detectors, signals polarized andmultiplexed on the transmitter side are mixed without being separated.In this case, when the modulation system is QPSK (Quadrature Phase ShiftKeying), the quadrupling is conducted in the computation units 3721 and3722, and the modulation signal nπ/4 (n=1, 3, 5, and 7) on thetransmission side becomes nπ (n=1, 3, 5, and 7).

For this reason, even when any rotation is applied in the opticaltransmission path, among the adjacent samples, as the complex number,the same phase is realized. For this reason, the conversion into thephase information is carried out in the computation units 3712 and 3722,and the computation is carried out in the computation units 3713 and3723, so that it is possible to calculate the phase rotation amount forone sample from the phase rotation amount (1−Z^(−2n)) among 2n samplessubjected to the 2× over sampling.

Then, through the addition of the H axis and the V axis in the adderunit 3730, it is possible to detect the frequency difference as the (2×)phase rotation amount. Even when the maximum frequency differencedecided in the system is input, n is decided so that the phase rotationamount among the 2n samples falls within −π to π. Also, in thecomputation of (1−Z^(−2n)) in the computation units 3713 and 3723, asthe case may be addition of ±2π is conducted so that the result fallswithin −π to π.

FIG. 38 is a block diagram showing a specific example of the frequencycompensator. The frequency compensator 3511 shown in FIGS. 35 and 36 isprovided, for example, as shown in FIG. 38, with an adder unit 3811, aremainder operation unit 3812, a delay element 3813, a computation unit3814, a multiplication unit 3815, and a multiplication unit 3816. Theadder unit 3811 adds the rotation control signal from the first DLF 513with a signal θ from the delay element 3813 and outputs the additionresult to the remainder operation unit 3812.

The remainder operation unit 3812 conducts a remainder operation on thesignal output from the adder unit 3811 while 2π is set as a divisor. Theremainder operation unit 3812 outputs the signal θ of the computationresult to the delay element 3813 and the computation unit 3814. Thedelay element 3813 delays the signal θ output from the remainderoperation unit 3812 by ½ symbols to be output to the adder unit 3811.

On the basis of the signal θ output from the remainder operation unit3812, the computation unit 3814 computes a rotator e^(j)θ for eachsample. The computation unit 3814 outputs the rotator e^(j)θ obtainedthrough the computation to the multiplication unit 3815 and themultiplication unit 3816.

The multiplication unit 3815 multiplies the signal on the H axis inputto the frequency compensator 3511 (complex number) by the rotator e^(j)θoutput from the computation unit 3814. The multiplication unit 3815outputs the multiplied signal on the H axis to the subsequent stage. Themultiplication unit 3816 multiplies the signal on the V axis input tothe frequency compensator 3511 (complex number) by the rotator e^(j)θoutput from the computation unit 3814. The multiplication unit 3816outputs the multiplied signal on the V axis to the subsequent stage.

In a case where the signals are input to the frequency compensator 3511in parallel, in order to process the N samples at the same time, Z⁻¹ inthe delay element 3813 is set as Z^(−N), and a rotator e^(jm)θ iscomputed in the m-th signal in the computation unit 3814. Z^(−N) isequivalent to one clock delay in the signal processing block.

(Configuration Example of Optical Transmission System)

FIG. 39 is a block diagram showing a specific example of the opticaltransmission system. As shown in FIG. 39, an optical transmission system3900 includes a transmitter 3910 and the digital coherent receiver 100.The transmitter 3910 transmits the optical signal via the opticaltransmission path including optical fibers 3911 to 3913 and opticalamplifiers 3921 and 3922 to the digital coherent receiver 100.

In the optical transmission system 3900, the waveform distortion of theoptical signal such as the wavelength dispersion generated in theoptical transmission path can be compensated in the digital coherentreceiver 100. For this reason, for the optical transmission path in theoptical transmission system 3900, a configuration without providing aDispersion Compensating Fiber (DCF) or the like for compensating thewavelength dispersion amount can also be adopted.

For this reason, cost reduction, space saving, and the like for theapparatus can be realized, and also the optical attenuation amount ofthe optical signal can be reduced through no provision of the DCF, sothat it is possible to reduce the number of the optical amplifiers canbe reduced. For this reason, the power consumption. Also, as comparedwith the optical waveform compensation circuit, the digital waveformcompensation circuit and the digital demodulation circuit in the digitalcoherent receiver 100 is superior in tracking property with respect tothe fluctuation of the transmission path distortion. For this reason,the configuration is also useful to the polarized wave multiplex systemwhere a high tracking property is demanded with respect to the polarizedwave.

(Overlap Type Fourier Transform Unit and Inverse Fourier Transform Unit)

In the Fourier transform units 811, 1311, and 2011 and the inverseFourier transform units 815, 1315, and 2014 which are shown in FIGS. 8,13, 14, and 20, the phase shift of Δτ in the time domain becomes therotator coefficient of exp^((j)ωΔτ⁾ in the frequency domain. For thisreason, the Fourier transform result of the input is multiplied by therotator coefficient, and the inverse Fourier transform is carried out torealize the phase shift.

However, if an attempt is made to realize the Fourier transform and theinverse Fourier transform by using the normal FFT, IFFT, DFT (DiscreteFourier Transform), or IDFT (Inverse DFT), discontinuous points may begenerated as the sample in which the phase is shifted circulates theFourier transform window after the inverse Fourier transform. Overlaptype Fourier transform unit and inverse Fourier transform unit forsolving this phenomenon will be described with reference to FIGS. 40 and41.

FIG. 40 is a block diagram showing specific example of the Fouriertransform unit and the inverse Fourier transform unit. For the Fouriertransform units 811, 1311, and 2011 and the inverse Fourier transformunits 815, 1315, and 2014 which are shown in FIGS. 8, 13, 14, and 20,for example, a circuit 4000 shown in FIG. 40 can be applied. The circuit4000 is provided with an input unit 4011, a FFT input frame generationunit 4012, a FFT processing unit 4013, a characteristic multiplicationunit 4014, an IFFT processing unit 4015, an IFFT output frame extractionunit 4016, and an output unit 4017.

Herein, the input data is set as 256 parallel signals, and a window sideof FFT and IFFT is set as 1024. The input data (time domain: 256samples) are input to the input unit 4011. The input unit 4011 buffersthe input data thus input and generates a frame composed of 512 samplesfor once in 2 clocks.

The input unit 4011 outputs the generated frame to the FFT input framegeneration unit 4012. Also, the input unit 4011 outputs a control signalincluding a frame generation timing to an internal counter of therespective blocks of the circuit 4000. In the subsequent stage of theinput unit 4011, the processing is carried out this frame and the framegeneration timing in the input unit 4011 are set as the unit.

The FFT input frame generation unit 4012 joins the one previous 512sample frames and the current 512 sample frames in the sample framesoutput from the input unit 4011 to generate a frame composed of 1024samples. The FFT input frame generation unit 4012 outputs the generatedframe to the FFT processing unit 4013.

The FFT processing unit 4013 transforms the frame output from the FFTinput frame generation unit 4012 into data in the frequency domain. TheFFT processing unit 4013 outputs the transformed frame to thecharacteristic multiplication unit 4014. The characteristicmultiplication unit 4014 respectively multiplies characteristicparameters for each frequency component with respect to the frequencycorresponding to the frame output from the FFT processing unit 4013 (for1024 frequencies). The characteristic parameters are input, for example,from an external area. The characteristic multiplication unit 4014outputs the multiplied frame to the IFFT processing unit 4015.

The IFFT processing unit 4015 transforms the frame output from thecharacteristic multiplication unit 4014 into data in the time domain.The IFFT processing unit 4015 outputs the transformed frame to the IFFToutput frame extraction unit 4016. In the vicinity of the frame outputfrom the IFFT processing unit 4015, the discontinuous points areincluded.

With regards the frame output from the IFFT processing unit 4015, theIFFT output frame extraction unit 4016 discards 256 samples in the backand forth, that is, the quarter of the window size each. If thediscontinuous points fall within an area for the discarding in the IFFToutput frame extraction unit 4016, the discontinuous points are notgenerated in the output obtained by joining the 512 samples which arenot discarded. The IFFT output frame extraction unit 4016 outputs theprocessed frame to the output unit 4017.

The output unit 4017 cuts the frame output from the IFFT output frameextraction unit 4016 (512 samples output every 2 samples) into 256samples each per 1 clock to be output as the parallel signal to thesubsequent stage.

FIG. 41 shows an operation of the circuit shown in FIG. 40. Referencenumeral 4110 in FIG. 41 denotes input data input to the input unit 4011.Reference symbol 4120 denotes N-th frame (FFT input frame) input to theFFT processing unit 4013. Reference symbol 4130 denotes (N+1)-th frameinput to the FFT processing unit 4013. Reference symbol 4140 denotes(N+2)-th frame input to the FFT processing unit 4013.

Reference symbol 4150 denotes N-th frame output from the IFFT processingunit 4015 (IFFT output frame). Reference symbol 4160 denotes (N+1)-thframe output from the IFFT processing unit 4015. Reference symbol 4170denotes (N+2)-th frame output from the IFFT processing unit 4015.Reference symbol 4171 is a frame discarded by the IFFT output frameextraction unit 4016.

Reference symbol 4180 denotes a frame in which the respective framesdenoted by Reference symbols 4150, 4160, and 4170 (except the framedenoted by reference numeral 4171) are joined by the IFFT output frameextraction unit 4016. In this manner, according to the circuit 4000 forconducting the overlap type FFT and IFFT, the generation of thediscontinuous points because of the phase shift can be avoided.

As described above, according to the digital coherent receiver, it ispossible to improve the communication quality.

The reception method described in the present embodiment can be realizedwhile a previously prepared program is executed by a computer such as apersonal computer or a work station. This program is recorded on acomputer-readable recording medium such as a hard disk, a flexible disk,a CD-ROM, an MO, or a DVD and, and the program is read out from therecording medium by the computer for the execution. Also, this programmay be a transmission medium which can be distributed via a network suchas the Internet.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment(s) of the presentinvention has(have) been described in detail, it may be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1-8. (canceled)
 9. A digital coherent receiving apparatus comprising: areceiver for receiving a light signal; a first oscillator for outputtinga local light signal of a fixed frequency; a hybrid unit for mixing thelocal light signal output from the first oscillator with the lightsignal received by the receiver; a second oscillator for outputting asampling signal of a sampling frequency, the second oscillator changingthe sampling frequency in response to an inputted frequency controlsignal; a converter for converting the mixed light signal into digitalsignal by sampling the mixed light signal synchronizing with thesampling signal; a waveform adjuster for adjusting a waveform distortionof the digital signal converted by the convertor; a phase detector fordetecting a phase signal of the digital signal adjusted by the waveformadjuster; a control signal output unit for outputting a frequencycontrol signal on the basis of the detected phase signal to the secondoscillator; a phase adjustor for adjusting a phase of the digital signaladjusted by the waveform adjustor; and a demodulator for demodulatingthe digital signal adjusted by the phase adjuster.
 10. The digitalcoherent receiving apparatus according to claim 9, wherein the controlsignal output unit outputs a frequency control signal generated byconverting the component of the detected phase into a frequencycomponent.
 11. The digital coherent receiving apparatus according toclaim 9, wherein the control signal output unit includes; a noisecanceller for cancelling a noise in the phase detected by the phasedetector, and a generator for generating the frequency control signal inreference to the phase after cancelling the noise by the noise cancellerand for outputting the frequency control signal to the secondoscillator, and the phase adjustor adjusts the phase of the digitalsignal on the basis of the phase after cancelling the noise by the phasedetector.
 12. The digital coherent receiving apparatus according toclaim 11, wherein the generator generates the frequency control signalby converting the phase component of the phase into a frequencycomponent.
 13. The digital coherent receiving apparatus according toclaim 9, wherein further comprising a noise canceller for cancelling anoise in the phase detected by the phase detector; and wherein the phaseadjustor adjusts the phase of the digital signal on the basis of thephase after cancelling the noise.
 14. The digital coherent receivingapparatus according to claim 13, wherein the control signal output unitoutputs a frequency control signal generated by converting the phasecomponent of the detected phase into a frequency component.
 15. Thedigital coherent receiving apparatus according to claim 9, wherein thephase detector includes; a plurality of equalizing filters forprocessing respectively that equalizes the digital signal by usingdifferent equalizing characteristics, a plurality of detecting unit fordetecting a plurality of the candidate phases of the digital signalprocessed by each of the equalizing filters, and selector for selectingone of the detected candidate phases as the detected phase.
 16. Thedigital coherent receiving apparatus according to claim 9, wherein thephase detector includes; a plurality of equalizing filters forprocessing respectively that equalizes the digital signal by usingdifferent equalizing characteristics, a plurality of detecting unit fordetecting a plurality of the candidate phases of the digital signalprocessed by each of the equalizing filters, a convertor for diversityconverting the plurality of the candidate phases, and a phase generatorfor generating the phase by combining the plurality of the candidatephases using a diversity combining method and for outputting the phase.17. A digital coherent receiving apparatus comprising: a firstoscillator for outputting a local light signal of a fixed frequency; ahybrid unit for mixing the local light signal output from the firstoscillator with the light signal received by the receiver; a secondoscillator for outputting a sampling signal of a sampling frequency, thesecond oscillator changing the sampling frequency in response to aninputted frequency control signal; a converter for converting the mixedlight signal into digital signal by sampling the mixed light signalsynchronizing with the sampling signal; a frequency difference detectordetects a difference between a frequency of the local light output fromthe first oscillator and a frequency of the light signal received by thereceiver in reference to the digital signal converted by the convertor;a frequency adjustor for adjusting a frequency of the digital signalconverted by the convertor on the basis of the detected difference; awaveform adjuster for adjusting a waveform distortion of the digitalsignal adjusted by the convertor on the basis of the difference detectedby the frequency difference detector; and a demodulator for demodulatingthe digital signal adjusted by the waveform adjustor.